Docs: Add user facing docs for running a separate Task Execution API-server

[kaxil]

In #43015, a new Task Execution API-server was added. This can be run with the Core Airflow APIs and can also be run separately.

To run all apis

airflow fastapi-api

# Alternate
airflow fastapi-api --apps all

# Alternate
airflow fastapi-api --apps core,execution

E.g to only run Task execution API

airflow fastapi-api --apps execution

The doc should contain details of our recommendation.

[Brijeshthummar02]

@kaxil i can work on it, could you show me the path to files, and where to add it also assign me.

[kaxil]

Hey @Brijeshthummar02 , AIP-72 is still under progress: https://github.com/orgs/apache/projects/405

This IMO would be need some background about AIP-72, so unless you have read it might be difficult.

Keeping it for myself or @ashb to work on it once we are done with that AIP.