sphinx: Use modern syntax for extlinks

This fixes the following build error:

  Command line: /opt/homebrew/opt/sphinx-doc/bin/sphinx-build -b man -d /private/tmp/extra-dir-55768274273/.doctrees-man -n -w /private/tmp/extra-dir-55768274273/.log docs/users_guide /private/tmp/extra-dir-55768274273
  ===> Command failed with error code: 2

  Exception occurred:
    File "/opt/homebrew/Cellar/sphinx-doc/6.0.0/libexec/lib/python3.11/site-packages/sphinx/ext/extlinks.py", line 101, in role
      title = caption % part
              ~~~~~~~~^~~~~~
  TypeError: not all arguments converted during string formatting

I tested on Sphinx-5.1.1 and Sphinx-6.0.0

Thanks for sterni for providing instructions about how to test using sphinx-6.0.0.

Fixes #22690 (closed)