Proposal: check if hardcoded URLs can be replaced with extlinks

[hoefling]

Subject: This PR adds checks for hardcoded URLs whether they can be replaced with an extlink. This is a feature similar to #9626, but for extlinks instead of intersphinx.

Feature or Bugfix

• Feature

Purpose

If a hardcoded link can be replaced with a crossref, sphinx will emit a warning like this:

pytest/doc/en/reference/reference.rst:127: WARNING: hardcoded link 'https://github.com/octocat' could be replaced by an extlink (try using ':user:`octocat`' instead)

Example output from running over Sphinx repository

/home/oleg.hoefling/projects/private/sphinx/doc/tutorial/narrative-documentation.rst:92: WARNING: hardcoded link 'https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html#hyperlink-targets' could be replaced by an extlink (try using ':duref:`hyperlink-targets`' instead)

Relates

• Proposal: add new builder that checks if hardcoded URLs can be replaced with crossrefs #9626

[tk0miya]

LGTM with nits.

[tk0miya]

Is there any reason for this priority? It's invoked on a very late step.
https://www.sphinx-doc.org/en/master/extdev/appapi.html#sphinx.application.Sphinx.add_transform

[tk0miya]

IMO, there is no special reason to invoke this feature earlier or later. So it would be better to put it on the normal priority: 500. I'll modify it after merging.

[hoefling]

@tk0miya thank you, those are very helpful! In fact, I'll apply those to #9626 as well.

[tk0miya]

Thank you for update. Merging now.