[RLlib][Docs] Added RLModule user-guide to the docs

[kouroshHakha]

Why are these changes needed?

Related issue number

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• 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/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[kouroshHakha]

@avnishn In the beginning of the doc I allude to your migration guide. So that need to get merged in for this make sense. Your doc will be cross linked to this one.

[kouroshHakha]

@ArturNiederfahrenhorst In the end of this doc I allude to the catalog user-guide (in the extension of existing RLModules section). You need to write your use-guide and then we will link it here.

[ArturNiederfahrenhorst]

@kouroshHakha sg

[ollie-iterators]

/ray/doc/source/rllib/rllib-rlmodule.rst:403: WARNING: Inline interpreted text or phrase reference start-string without end-string.

[avnishn]

publishing for now

[avnishn]

leaving additional comments from offline convor

[richardliaw]

Hey, we should be testing codeblocks in CI and not using code-blocks. Can you fix? You can take a look at Tune for an example of what to do.

[maxpumperla]

like Richard mentioned, let's make all these literalinclude statements. Otherwise these code blocks risk being outdated quickly. RLlib already has a doc_code folder for that purpose. Alternatively, use an example or test folder in rllib, but make sure it's all tested. Thanks!