Add documentation for hybrid executor and ungate the feature

[o-nikolas]

Publicly "release" the hybrid executor feature, including:

• Documentation
• Remove feature gating
  • Remove mocks that disabled the feature gate for testing purposes
  • Removed tests for feature gate itself
• Possibly a changelog/whats new?

A rendered view of the docs can be seen below:

---

^ Add meaningful description above
Read the Pull Request Guidelines for more information.
In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in newsfragments.

[o-nikolas]

@eladkal Do I need to write any change log or what's new blurbs for this one? Let me know what you think.

Also maybe a post for https://airflow.apache.org/blog/ (But that would be outside of this PR of course).

[eladkal]

@eladkal Do I need to write any change log or what's new blurbs for this one? Let me know what you think.

Also maybe a post for https://airflow.apache.org/blog/ (But that would be outside of this PR of course).

I don't think it requires seperated blog entry but that is always welcome if you want to!
I think we do want newsfragment for segnificant feature.

[ferruzzi]

Nice. The pros and cons are a nice touch, and thanks for including the rendered version of the docs.

[o-nikolas]

@eladkal Do I need to write any change log or what's new blurbs for this one? Let me know what you think.
Also maybe a post for https://airflow.apache.org/blog/ (But that would be outside of this PR of course).

I don't think it requires seperated blog entry but that is always welcome if you want to! I think we do want newsfragment for segnificant feature.

Thanks @eladkal! I may write a blog in the coming week, but for now I'll just add a newsfragment to this PR.

[o-nikolas]

@eladkal Do I need to write any change log or what's new blurbs for this one? Let me know what you think.
Also maybe a post for https://airflow.apache.org/blog/ (But that would be outside of this PR of course).

I don't think it requires seperated blog entry but that is always welcome if you want to! I think we do want newsfragment for segnificant feature.

Thanks @eladkal! I may write a blog in the coming week, but for now I'll just add a newsfragment to this PR.

I added a fragment @eladkal, let me know what you think

[potiuk]

LGTM

[kaxil]

Can this be merged @o-nikolas ?

[o-nikolas]

Can this be merged @o-nikolas ?

Hey! I was waiting for @syedahsn's executor metrics PR to be merged first. But he can always circle back and update these docs as part of his PR I suppose.