Use canonical RFC Editor URLs independently of Docutils

[Darlokt]

Purpose

Hej,
this updates the :rfc: role to use the canonical RFC Editor URL.

https://www.rfc-editor.org/info/rfcNNNN/

I was double checking if #14681 was enough to really fix all CI, as mentioned in #14677 but with new changes landing in upstream Docutils, this was no longer true, so here now the fixes to the newest 4 CI failures.

Sphinx currently provides its own rfc_base_url but uses Docutils Inliner.rfc_url template for the remaining path.
This makes generated URLs depend on the installed Docutils version: released version uses rfc%d.html, while current Docutils development versions use rfc%d/.
So, the same Sphinx version can generate different RFC URLs depending only on the installed Docutils version.
Combining the new RFC Editor base URL with the template from a released Docutils version would also produce /info/rfcNNNN.html, which is not a valid RFC Editor URL.

To fix this cleanly and decouple Sphinx a bit, Sphinx now constructs the rfcNNNN/ portion itself. This keeps RFC URLs consistent across supported and development Docutils versions while continuing to respect the configurable rfc_base_url.

The default base URL is changed from
https://datatracker.ietf.org/doc/html/ to the canonical
https://www.rfc-editor.org/info/.

Focused regression tests cover plain RFC references, section anchors, empty anchors, custom base URLs, HTML output, and LaTeX output.
The environment version is incremented so existing cached doctrees are rebuilt with the new URLs.
The declared docutils>=0.21,<0.23 dependency range is unchanged.

References

• Closes Use canonical URL for RFCs #14659
• Alternative implementation to Use canonical rfc-editor.org URL for the :rfc: role #14662. But this version here explicitly decouples Sphinx’s RFC URL construction from Docutils and adds focused regression and invalidates existing build environments.
• Related to Fix docutils doctest table width compatibility #14681 only through the moving Docutils-HEAD CI lane. The RFC URL change is independent of its doctest and table-width compatibility fixes.

AI Disclosure

OpenAI Codex assisted with reviewing the implementation, analyzing compatibility with released and development Docutils versions. All AI-assisted work was reviewed and understood by me.

[Darlokt]

The failures are flakey tests and the missing changes from #14681. I keep them separate, as they solve different issues, merging the two would resolve the Docutils HEAD failures and the LaTeX failures.