Add more information about developer tooling for docs contributions

[peytondmurray]

Why are these changes needed?

This PR expands the guidelines for contributing to docs by adding a section about esbonio, a language server for sphinx.

Additionally, docs.ipynb has been converted to docs.md after a discussion with @can-anyscale; there was never any python code being executed in this doc, so there's no reason to have the added complexity of it being a notebook. I also added .virtual_documents/ to the .gitignore for those times when we do need to modify a notebook with jupyterlab.

Related issue number

No issue, but addressed doc-infra need to improving documentation editing and providing a syntax assistant.

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 :(

[can-anyscale]

CC: @angelinalg if you want to review the content