Docs: Make the providers operators/hooks reference much more usable

[potiuk]

The providers operators/hooks reference contained only top-level list of
groups of providers, which make them less-usable than they could be as
the users did not see at this page links to particular operators/hooks,
it was not really visible what is "available" (discoverability) and
the more detailed "Service" and "Transfer" pages are not really
readable enough to give "at a glance" overview what is available.

This change improves that, removes the repeated multiple times
"operators and hooks" which was kind of annoying, and increases
the TOC-level to 3 giving a nice overview of all available and
exposed operator and hooks.

---

^ Add meaningful description above

Read the Pull Request Guidelines for more information.
In case of fundamental code change, 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 UPDATING.md.

[potiuk]

I have a feeling that what I propose is WAY more usable:

Compare this (old one):

With this (new one):

[github-actions]

The PR is likely ready to be merged. No tests are needed as no important environment files, nor python files were modified by it. However, committers might decide that full test matrix is needed and add the 'full tests needed' label. Then you should rebase it to the latest main or amend the last commit of the PR, and push it with --force-with-lease.