Add keywords, move indicators API to its own page

[aulemahal]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes Create subrealms/categories, atmos.temperature, atmos.precipitation, atmos.wind, atmos.mixed, etc, for temperature, precip, wind and mixed indicators #1433
• Tests for the changes have been added (for bug fixes / features)
  • (If applicable) Documentation has been added / updated (for bug fixes / features)
• CHANGES.rst has been updated (with summary of main changes)
  • Link to issue (:issue:number) and pull request (:pull:number) has been added

What kind of change does this PR introduce?

• Changes how the "keyword" attribute is inherited between indicators to inherit by concatenating.
• Add basic keywords to all subclasses
• Move the indicators API to its own page to lower loading times (it's still slow, but a lot more bearable IMO)

Does this PR introduce a breaking change?

No.

Other information:

[Zeitsperre]

Makes a bunch of sense and is indeed quicker to load!

[Zeitsperre]

This PR just needs its Changes.rst updated and we can merge!