docs: Add user guide for Dekaf (#1573)

* Add user guide for Dekaf

* Add user guide for Dekaf

site/docs/guides/dekaf_reading_collections_from_kafka.md

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+# Connecting to Estuary Flow from Kafka using Dekaf
+
+**Dekaf** is Estuary Flow's Kafka API compatibility layer, allowing consumers to read data from Estuary Flow collections
+as if they were Kafka topics. Additionally, Dekaf provides a schema registry API for managing schemas. This guide will
+walk you through the steps to connect to Estuary Flow using Dekaf and its schema registry.
+
+## Overview
+
+- **Collections** represent datasets within Estuary Flow. All captured documents are written to a collection, and all
+  materialized documents are read from a collection.
+- **Dekaf** enables you to interact with these collections as though they were Kafka topics, providing seamless
+  integration with existing Kafka-based tools and workflows.
+
+## Key Features
+
+- **Kafka Topic Emulation**: Access Estuary Flow collections as if they were Kafka topics.
+- **Schema Registry Emulation**: Manage and retrieve schemas assigned to Estuary Flow collections, emulating Confluent's
+  Schema Registry.
+
+## Connection Details
+
+To connect to Estuary Flow via Dekaf, you need the following connection details:
+
+- **Broker Address**: `dekaf.estuary.dev`
+- **Schema Registry Address**: `https://dekaf.estuary.dev`
+- **Security Protocol**: `SASL_SSL`
+- **SASL Mechanism**: `PLAIN`
+- **SASL Username**: `{}`
+- **SASL Password**: Estuary Refresh Token (Generate your token in
+  the [Estuary Admin Dashboard](https://dashboard.estuary.dev/admin/api))
+- **Schema Registry Username**: `{}`
+- **Schema Registry Password**: The same Estuary Refresh Token as above
+
+## How to Connect to Dekaf
+
+### 1. Generate an Estuary Refresh Token:
+
+1. Log in to the Estuary Admin Dashboard.
+2. Navigate to the section where you can generate tokens.
+3. Generate a new refresh token and note it down securely.
+
+### 2. Set Up Your Kafka Client
+
+Configure your Kafka client using the connection details provided.
+
+#### Example Kafka Client Configuration
+
+Below is an example configuration for a Kafka client using Python’s `kafka-python` library:
+
+```python
+from kafka import KafkaConsumer
+
+# Configuration details
+conf = {
+    'bootstrap_servers': 'dekaf.estuary.dev:9092',
+    'security_protocol': 'SASL_SSL',
+    'sasl_mechanism': 'PLAIN',
+    'sasl_plain_username': '{}',
+    'sasl_plain_password': 'Your_Estuary_Refresh_Token',
+    'group_id': 'your_group_id',
+    'auto_offset_reset': 'earliest'
+}
+
+# Create Consumer instance
+consumer = KafkaConsumer(
+    'your_topic_name',
+    bootstrap_servers=conf['bootstrap_servers'],
+    security_protocol=conf['security_protocol'],
+    sasl_mechanism=conf['sasl_mechanism'],
+    sasl_plain_username=conf['sasl_plain_username'],
+    sasl_plain_password=conf['sasl_plain_password'],
+    group_id=conf['group_id'],
+    auto_offset_reset=conf['auto_offset_reset'],
+    enable_auto_commit=True,
+    value_deserializer=lambda x: x.decode('utf-8')
+)
+
+# Poll for messages
+try:
+    for msg in consumer:
+        print(f"Received message: {msg.value}")
+except KeyboardInterrupt:
+    pass
+finally:
+    consumer.close()
+```
+
+You can also use [kcat](https://github.com/edenhill/kcat) (formerly known as kafkacat) to test reading messages from an
+Estuary Flow collection as if it were a Kafka topic.
+
+```shell
+kcat -C \
+    -X broker.address.family=v4 \
+    -X security.protocol=SASL_SSL \
+    -X sasl.mechanism=PLAIN \
+    -X sasl.username="{}" \
+    -X sasl.password="Your_Estuary_Refresh_Token" \
+    -b dekaf.estuary.dev:9092 \
+    -t "full/nameof/estuarycolletion" \
+    -p 0 \
+    -o beginning \
+    -s avro \
+    -r https://{}:{Your_Estuary_Refresh_Token}@dekaf.estuary.dev
+```