Add docs about Scheduler HA, how to use it and DB requirements

[ashb]

This PR documents (from a user point of view) how Scheduler HA #10956 works, what the requirements are, and what config options there are that users might want to configure.

[ashb]

This is only a start, I'll add more. What would else would people like to see?

[XD-DENG]

A few typo fixes to suggest.

[turbaszek]

Suggested change

	Consul for instance) we have kept the "operational surface area" to a minimum.
	Consul for instance) we have kept the "operational surface area" to a minimum. For more information on architectural choices check `Airflow Improvement Proposal 15 <https://cwiki.apache.org/confluence/pages/viewpage.action?pageId=103092651>`_.

[ashb]

Hmmm, not sure about this one -- I like the idea, but I think I'd rather the docs be the source of truth rather than linking to the AIP.

I might link to the AIP for now though.

[turbaszek]

Look good, should we put a stronger emphasis on the fact that serialization is required for HA?

[mik-laj]

Can you also add some documentation on the database setup page? If users know about this requirement in advance, they will be able to select these databases when creating an instance. Otherwise, users will rely on the default values provided by the cloud provider.
https://airflow.readthedocs.io/en/latest/howto/initialize-database.html

[ashb]

Can you also add some documentation on the database setup page? If users know about this requirement in advance, they will be able to select these databases when creating an instance. Otherwise, users will rely on the default values provided by the cloud provider.
https://airflow.readthedocs.io/en/latest/howto/initialize-database.html

I've cross-linked it from there

[ryw]

@ashb i think you should merge this as is - will be easier for people to collaborate on improving it w/ small PRs vs review comments.

[github-actions]

The Workflow run is cancelling this PR. It has some failed jobs matching ^Pylint$,^Static checks,^Build docs$,^Spell check docs$,^Backport packages$,^Provider packages,^Checks: Helm tests$,^Test OpenAPI*.