[Doc] Use sphinx_book_theme

[simon-mo]

Why are these changes needed?

This PR updates the theme for use Sphinx Book Theme. A full rendering can be found here

Note:

• Breadcrumbs are gone.

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/latest/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failure rates at https://ray-travis-tracker.herokuapp.com/.
  • Running few sanity checks:
    • Google Analytics still works
    • Read the docs build passes. It should appear in https://simon-ray.readthedocs.io/en/doc-new-theme/
    • Make sure version warnings still appear.

[simon-mo]

These should replace our custom "open an issue" button.

[richardliaw]

nice >:)

[simon-mo]

Note: We need to wait for executablebooks/sphinx-book-theme#198 to be merged and released.

[ericl]

@simon-mo several of the rllib tables look funny in the new theme, are there are fixes you can make to the config to make them look more compact?

[richardliaw]

I think the fix there is to do it via the scss; could look into it later this weekend.

[richardliaw]

TODO before merge:

1. fix the tables as mentioned by eric
2. make sure centering works

We may want to just override the scss file with the corresponding changes for 1.0.

[simon-mo]

@richardliaw @ericl I updated the centering and table.

• Full Preview: https://anyscale-ray-doc-assets.s3-us-west-2.amazonaws.com/index.html

Image Centering

Table Compact

[richardliaw]

last nit here @simon-mo