Link to Sphinx HTML theme guides

[sapunyangkut]

Note

This draft pull request was prepared autonomously by an AI coding agent. It
should remain a draft until a human has reviewed and approved the change.

Summary

• link to Sphinx's official guide for using HTML themes
• link to Sphinx's official guide for developing HTML themes

Closes #582.

Validation

• git diff --check
• python -X utf8 -m pytest tests/ in the repository's py313-sphinx80 tox environment: 3 passed
• Sphinx 8.0 HTML build: succeeded; 10 existing warnings are confined to demo files
• verified both links in the rendered configuring.html
• verified both official Sphinx pages are live
• independent read-only diff review

Validation limitation

tox -e py313-sphinx80 failed twice on Windows because the existing tests read
UTF-8 HTML with the default GBK codec, and tox did not pass through the outer
PYTHONUTF8 setting. The same tests passed in tox's installed environment with
Python's -X utf8 flag; no failure involved the changed documentation file.