chore: remove Circle-CI, fix GHA

[dblock]

:rocket: Why this change?

Build goes to green again.

• Removes Circle-CI. Looks dead? Unused?
• Moves smoke tests to GitHub Actions.
• Run tests/lints/etc. on pull_request.
• Renames jobs to tests (plural) to be consistent.
• Locks sphinx doc dependencies to those compatible with v4.3.0.
• https://github.com/ungrim97/Dredd-Hooks doesn't have functioning CI anymore (Travis CI is RIP), so links in docs removed
• Fixes yarn docs:lint, but loses some anchors that no longer work and I couldn't find easy replacements for

See https://github.com/dblock/dredd/actions for everything passing.

:memo: Related issues and Pull Requests

#2294

:white_check_mark: What didn't I forget?

• [x] To write docs
• [x] To write tests
• [x] To put Conventional Changelog prefixes in front of all my commits and run npm run lint