Recover Kafka notes and make reference to deployment notes (#13880)

Recover changes from #13834 and point to more detailed instructions in
the shared Kafka README file.

Author: armenzg

develop-docs/backend/application-domains/kafka.mdx

@@ -3,4 +3,34 @@ title: Kafka consumers
 sidebar_order: 60
 ---
 
-Visit https://github.com/getsentry/ops/tree/master/shared_config/kafka for a full, in-depth step-by-step guide.
+## Create a new Kafka topic
+
+1. Add the topic to the `KAFKA_TOPIC_TO_CLUSTER` in [src/sentry/conf/server.py](https://github.com/getsentry/sentry/blob/master/src/sentry/conf/server.py):
+  * e.g. `subscription-results-eap-items`
+2. Add the topic to `Topic` in [src/sentry/conf/types/kafka_definition.py](https://github.com/getsentry/sentry/blob/master/src/sentry/conf/types/kafka_definition.py)
+
+## Define or re-use a processing strategy
+
+In most cases a [Streaming Factory](https://getsentry.github.io/arroyo/getstarted.html#create-a-streaming-consumer) is what you want to when defining a consumer (see next section). You can find examples of it in [Sentry's code base](https://github.com/search?q=repo%3Agetsentry%2Fsentry+%28ProcessingStrategyFactory&type=code).
+
+## Define a new Kafka consumer
+
+1. Add a new entry in the `KAFKA_CONSUMERS` key in
+   [src/sentry/consumers/__init__.py](https://github.com/getsentry/sentry/blob/master/src/sentry/consumers/__init__.py):
+```python
+KAFKA_CONSUMERS = {
+    "<your_topic_str_here>": {
+        "topic": Topic.YOUR_TOPIC,
+        "strategy_factory": "sentry_package_defining_your_strategy_factory_class",
+    }
+}
+```
+2. You may need optional properties (e.g. `click_options`, you will need to research them by looking at [ConsumerDefinition](https://github.com/getsentry/sentry/blob/master/src/sentry/conf/types/kafka_definition.py)'s code.
+
+3. Make sure you can run it: `sentry run consumer <your_topic>`
+4. You may need to add some devserver options [here](https://github.com/getsentry/sentry/blob/master/src/sentry/runner/commands/devserver.py).
+4. Add tests for your consumer
+
+## Deployment
+
+Visit the Ops repo and search for `shared_config/kafka/README.md` for a full, in-depth step-by-step guide.