[RLlib] Remove CLI from docs (soon to be deprecated and replaced by python API).

[sven1977]

Remove CLI from docs (soon to be deprecated; replaced by python API).

Reasons, why we are deprecating the CLI:

• Simplification: Maintaining one python API is simpler than maintaining 2 top-level user APIs
• The CLI - even though it is a nice tool to quickly run a first RLlib experiment - has users quickly run into roadblocks when relying on this API.
• Instead, users should immediately learn the "only" valid API, which will be the python API of RLlib.
• To keep some level of command-line based ease-of-access, all our example scripts and tuned_examples are already directly runnable as python scripts with unified command line options.

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 :(

[simonsays1980]

LGTM. Very nice cleanup!

[simonsays1980]

Awesome, this relict is also gone, now!

[simonsays1980]

Remove one "the" ;)