ci: add dependabot and build badge

.github/dependabot.yml

@@ -0,0 +1,30 @@
+version: 2
+updates:
+
+  # GitHub actions
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    reviewers:
+      - "elastic/observablt-ci"
+    schedule:
+      interval: "weekly"
+      day: "sunday"
+      time: "22:00"
+    groups:
+      github-actions:
+        patterns:
+          - "*"
+
+  # GitHub composite actions
+  - package-ecosystem: "github-actions"
+    directory: "/.github/workflows/bootstrap"
+    schedule:
+      interval: "weekly"
+      day: "sunday"
+      time: "22:00"
+    reviewers:
+      - "elastic/observablt-ci"
+    groups:
+      github-actions:
+        patterns:
+          - "*"

README.md

@@ -1,3 +1,5 @@
+[![Pull Request Validation](https://github.com/elastic/elastic-otel-dotnet/actions/workflows/ci.yml/badge.svg)](https://github.com/elastic/elastic-otel-dotnet/actions/workflows/ci.yml)
+
 # Elastic OpenTelemetry Distribution
 
 This is Elastic's distribution of OpenTelemetry for .NET.