Modernize Sphinx configuration

[stefan6419846]

Short description

This PR resolves the build warnings when building the docs with the latest Sphinx versions. Additionally, some missing issue/PR links have been added.

Proposed changes

• Set language code to English due to:

  WARNING: Invalid configuration value found: 'language = None'. Update your configuration to a valid language code. Falling back to 'en' (English).

• Switch to the new intersphinx_mapping format due to:

  WARNING: The pre-Sphinx 1.0 'intersphinx_mapping' format is deprecated and will be removed in Sphinx 8. Update to the current format as described in the documentation. Hint: "intersphinx_mapping = {'': ('https://docs.python.org/', None)}".https://www.sphinx-doc.org/en/master/usage/extensions/intersphinx.html#confval-intersphinx_mapping

• Add a newline to resolve

  /home/docs/checkouts/readthedocs.org/user_builds/xhtml2pdf/checkouts/latest/docs/source/release-notes.rst:163: WARNING: Line block ends without a blank line.

• Use the issue/PR number for the issue and PR directive labels to fix

  Exception occurred:
    File "/home/docs/checkouts/readthedocs.org/user_builds/xhtml2pdf/envs/latest/lib/python3.9/site-packages/sphinx/ext/extlinks.py", line 108, in role
      title = caption % part
  TypeError: not all arguments converted during string formatting
  

  This is documented in the official docs at https://www.sphinx-doc.org/en/master/usage/extensions/extlinks.html: The caption (the second tuple element) needs to always be either None to use the full URL or a string which contains the placeholder %s exactly once.

  Additionally, the directives have been fixed to use the format from the official docs inside the actual RST.

Resolved issues

Fixes: #710

[timobrembeck]

Amazing, thanks a lot for the quick fix and the improvements! 💪