Use canonical rfc-editor.org URL for the :rfc: role

[sidsri14]

Purpose

Change the default rfc_base_url so the :rfc: role links to the
canonical RFC Editor location
(https://www.rfc-editor.org/rfc/rfcNNNN.html) instead of
https://datatracker.ietf.org/doc/html/rfcNNNN.html.

RFCs are officially published at rfc-editor.org, and datatracker has started
emitting a canonical link to rfc-editor.org for its RFC HTML pages
(see ietf-tools/datatracker#11481), confirming the rfc-editor.org URLs as
canonical.

• Changed the rfc_base_url default in sphinx/environment/__init__.py
• Updated the affected URL assertions in tests/test_markup/test_markup.py
  and tests/test_builders/test_build_html_5_output.py
• Added a CHANGES.rst entry and an AUTHORS.rst entry

Note: this only changes the default; users can still override
rfc_base_url explicitly.

References

Closes #14659

AI disclosure

This change was prepared with the assistance of Claude Code (Anthropic).
All changes were reviewed and verified locally before submission.

[holloway]

fwiw the canonical RFC location is https://www.rfc-editor.org/info/rfcN/. Note the 'info' prefix, that this doesn't have a .html suffix, and it's got a trailing slash.

Routes like https://www.rfc-editor.org/rfc/rfc2324.html have an HTTP header canonical url of https://www.rfc-editor.org/info/rfc2324/ etc.

[sidsri14]

Done — thanks for the pointer, @holloway. I've switched to the canonical https://www.rfc-editor.org/info/rfcN/ format:

• rfc_base_url now defaults to https://www.rfc-editor.org/info/
• the role builds rfcN/ links (no .html, trailing slash) instead of relying on docutils' rfc%d.html format, e.g. :rfc:2324 → `https://www.rfc-editor.org/info/rfc2324/` and `:rfc:`2324#section-1 → https://www.rfc-editor.org/info/rfc2324/#section-1.

Tests updated and passing.