Add a README/Doc file regarding the usage of Infra Alert Config Resource

docs/resources/infra_alert_config.md

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+# Infrastructure Alert Configuration Resource
+
+Management of Infrastructure alert configurations (Infrastructure Smart Alerts).
+
+API Documentation: <https://instana.github.io/openapi/#tag/Infrastructure-Alert-Configuration>
+
+The ID of the resource which is also used as unique identifier in Instana is auto generated!
+
+## Example Usage
+
+```hcl
+resource "instana_infra_alert_config" "example" {
+  name = "test-alert"
+  description = "test-alert-description"
+  alert_channels {
+    warning = ["alert-channel-id-1"]
+    critical = ["alert-channel-id-2"]
+  }
+  group_by = ["metricId"]
+  granularity = 600000
+  tag_filter = "host.fqdn@na STARTS_WITH 'fooBar'"
+
+  rules {
+    generic_rule {
+      metric_name = "cpu\\.(nice|user|sys|wait)"
+      entity_type = "host"
+      aggregation = "MEAN"
+      cross_series_aggregation = "SUM"
+      regex = true
+      threshold_operator = ">="
+
+      threshold {
+        
+        critical {
+          static {
+            value = 5.0
+          }
+        }
+        
+        warning {
+          static {
+            value = 3.0
+          }
+        }
+        
+      }
+    }
+  }
+
+  time_threshold {
+    violations_in_sequence {
+      time_window = 600000
+    }
+  }
+
+  custom_payload_field {
+    key = "test1"
+    value = "foo"
+  }
+
+  custom_payload_field {
+    key = "test2"
+    dynamic_value {
+      key = "dynamic-value-key"
+      tag_name = "dynamic-value-tag-name"
+    }
+  }
+}
+```
+
+## Argument Reference
+
+* `name` - Required - The name for the infrastructure alert configuration
+* `description` - Required - The description text of the infrastructure alert config
+* `alert_channels` - Optional - Set of alert channel IDs associated with the severity. [Details](#alert-channels-reference)
+* `group_by` - Optional - The grouping tags used to group the metric results.
+* `granularity` - Optional - default `600000` - The evaluation granularity used for detection of violations of the defined threshold. In other words, it defines the size of the tumbling window used. Allowed values: `300000`, `600000`, `900000`, `1200000`, `800000`
+* `tag_filter` - Optional - The tag filter of the global application alert config. [Details](#tag-filter-argument-reference)
+* `rules` - Required - A list of rules where each rule is associated with multiple thresholds and their corresponding severity levels. This enables more complex alert configurations with validations to ensure consistent and logical threshold-severity combinations. [Details](#rules-argument-reference)
+* `time_threshold` - Required - Indicates the type of violation of the defined threshold.  [Details](#time-threshold-argument-reference)
+* `custom_payload_filed` - Optional - An optional list of custom payload fields (static key/value pairs added to the event).  [Details](#custom-payload-field-argument-reference)
+
+### Alert Channels Reference
+
+* `warning` - Optional - List of alert channel IDs associated with the warning severity 
+* `critical` - Optional - List of alert channel IDs associated with the critical severity
+
+### Tag Filter Argument Reference
+The **tag_filter** defines which entities should be included into the application. It supports:
+
+* logical AND and/or logical OR conjunctions whereas AND has higher precedence then OR
+* comparison operators EQUALS, NOT_EQUAL, CONTAINS | NOT_CONTAIN, STARTS_WITH, ENDS_WITH, NOT_STARTS_WITH, NOT_ENDS_WITH, GREATER_OR_EQUAL_THAN, LESS_OR_EQUAL_THAN, LESS_THAN, GREATER_THAN
+* unary operators IS_EMPTY, NOT_EMPTY, IS_BLANK, NOT_BLANK.
+
+The **tag_filter** is defined by the following eBNF:
+
+```plain
+tag_filter                := logical_or
+logical_or                := logical_and OR logical_or | logical_and
+logical_and               := primary_expression AND logical_and | primary_expression
+primary_expression        := comparison | unary_operator_expression
+comparison                := identifier comparison_operator value | identifier@entity_origin comparison_operator value | identifier:tag_key comparison_operator value | identifier:tag_key@entity_origin comparison_operator value
+comparison_operator       := EQUALS | NOT_EQUAL | CONTAINS | NOT_CONTAIN | STARTS_WITH | ENDS_WITH | NOT_STARTS_WITH | NOT_ENDS_WITH | GREATER_OR_EQUAL_THAN | LESS_OR_EQUAL_THAN | LESS_THAN | GREATER_THAN
+unary_operator_expression := identifier unary_operator | identifier@entity_origin unary_operator
+unary_operator            := IS_EMPTY | NOT_EMPTY | IS_BLANK | NOT_BLANK
+tag_key                   := identifier | string_value
+entity_origin             := src | dest | na
+value                     := string_value | number_value | boolean_value
+string_value              := "'" <string> "'"
+number_value              := (+-)?[0-9]+
+boolean_value             := TRUE | FALSE
+identifier                := [a-zA-Z_][\.a-zA-Z0-9_\-/]*
+```
+
+### Rules Argument Reference
+
+* `generic_rule` - Required - A generic rule based on custom aggregated metric [Details](#generic-rule-argument-reference)
+
+#### Generic Rule Argument Reference 
+
+* `metric_name` - Required - The metric name of the infrastructure alert rule
+* `entity_type` - Required - The entity type of the infrastructure alert rule
+* `aggregation` - Required - The aggregation function of the infra alert rule. Supported values `MEAN`, `MAX`, `MIN`, `P25`, `P50`, `P75`, `P90`, `P95`, `P98`, `P99`, `SUM`, `PER_SECOND`
+* `cross_series_aggregation` - Required - Cross-series aggregation function of the infra alert rule. Supported values `MEAN`, `MAX`, `MIN`, `P25`, `P50`, `P75`, `P90`, `P95`, `P98`, `P99`, `SUM`
+* `regex` - Optional - Indicates if the given metric name follows regex pattern or not
+* `threshold_operator` - Required - The operator which will be applied to evaluate the threshold
+* `threshold` - Required - Indicates the type of threshold associated with given severity this alert rule is evaluated on.  [Details](#threshold-rule-argument-reference)
+* `time_threshold` - Required - Indicates the type of violation of the defined threshold.  [Details](#time-threshold-argument-reference)
+
+#### Threshold Rule Argument Reference
+
+At least one of the elements below must be configured
+
+* `warning` - Optional - Threshold associated with the warning severity [Details](#threshold-argument-reference)
+* `critical` - Optional - Threshold associated with the critical severity [Details](#threshold-argument-reference)
+
+##### Threshold Argument Reference
+
+* `static` - Required - Static threshold definition. [Details](#static-threshold-argument-reference)
+
+###### Static Threshold Argument Reference
+
+* `operator` - Required - The operator which will be applied to evaluate the threshold. Supported values: `>`, `>=`, `<`, `<=`
+* `value` - Optional - The value of the static threshold
+
+### Time Threshold Argument Reference
+
+* `violations_in_sequence` - Required - Time threshold base on violations in sequence. [Details](#violations-in-sequence-time-threshold-argument-reference)
+
+#### Violations In Sequence Time Threshold Argument Reference
+
+* `time_window` - Required - The time window if the time threshold
+
+### Custom Payload Field Argument Reference
+
+* `key` - Required - The key of the custom payload field
+* `value` - Optional - The static string value of the custom payload field. Either `value` or `dynamic_value` must be defined.
+* `dynamic_value` - Optional - The dynamic value of the custom payload field [Details](#dynamic-custom-payload-field-value). Either `value` or `dynamic_value` must be defined.
+
+#### Dynamic Custom Payload Field Value
+* `key` - Optional - The key of the tag which should be added to the payload
+* `tag_name` - Required - The name of the tag which should be added to the payload