Replace deprecated readthedocs GHA with built-in PR build integration

[msto]

Summary

The readthedocs.yml workflow currently uses the readthedocs/actions/preview
action to post documentation preview links on PRs that touch docs/**. This
action is deprecated. From the repository README:

This action is deprecated and it shouldn't be used. This feature was
included in the Read the Docs application itself.

Proposed change

Remove .github/workflows/readthedocs.yml and instead configure Read the Docs'
native pull request build integration, which provides build status and preview
links directly. The setup steps are documented here:

https://docs.readthedocs.com/platform/stable/visual-diff.html#show-build-overview-in-pull-requests

The key steps would be:

1. Enable "Build pull requests" in the Read the Docs project settings under
   Settings → General → Build pull requests
2. Optionally enable Visual Diffs for side-by-side doc change previews
3. Delete .github/workflows/readthedocs.yml

This gives us the same (or better) PR experience with one fewer GHA workflow to
maintain.

Context

Noticed while pinning GHA hashes in #284. Happy to submit a follow-up PR for
this if it sounds good!

🤖 Generated with Claude Code