[docs] RLlib concepts consolidation, user guide, RL conf prep

[maxpumperla]

Preview: RLlib docs

• Two key concepts pages merged into one (addresses [Docs] RLlib: Only have one "Concepts" page  #21832 partly, but need more concepts, still)
• Main navigation was cleaned up and organised into a user guide. Also still needs more love, but the project is hopefully a little easier to navigate.
• TOC page was partly merged into landing page, rest was removed (no value over docs nav and other content)
• We now have a central announcement include (header and footer) to advertise our conference (as per input from Asawari). Sticky banner on the lower right can also be configured.

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.

[sven1977]

Looks really cool! Thanks for cleaning this up and reorganizing the content. Just a few nits, then let's merge. :)

[sven1977]

Thanks for the fixes! Waiting for tests to pass ...

[simon-mo]

Because this contain doc only changes and passed lint + readthedocs build, and time sensitive, i'm merging this without macOS build (which is under provisioned and not relevant to current PR).