[Doc] Turn on nitpicky mode

[peytondmurray]

Why are these changes needed?

This PR prevents developers from breaking internal sphinx references by causing the docs build to fail if a broken reference is found.

Previously, broken references would simply be rendered as inline code but now they will raise an exception during the build process. It will be the responsibility of the developer who changes/adds/removes APIs or documentation to ensure the documentation is updated accordingly.

Furthermore, single backticks (`) will now default to just being rendered as code; previously Sphinx considered anything in single backticks a reference. Most developers who touch documentation are familiar with markdown rather than RST, and use single backticks to denote code although it is not valid RST syntax for code (RST uses double backticks). As a result, there are many many places in the documentation that use single backticks to mean code, and it would be impractical to expect contributors to adhere strictly to RST syntax when everyone is used to markdown. So instead we modify the default_role to eliminate this problem entirely, and treat anything in single backticks as code.

Related issue number

Closes #39658. Currently blocked by #46102; will mark this as ready once that PR is merged.

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]

thanks for working on this

[can-anyscale]

[can-anyscale]

this is breaking doc build in master i believe https://buildkite.com/ray-project/postmerge/builds/5103#01904cb8-8310-456d-bcd2-146a52e30d93; reverting to unblock