Fix current version title links on 404 pages

[Herrtian]

Fixes #9.

Summary

• add a Read the Docs-only script on 404 pages to point logo/title links at the current version root
• leave existing resource, search, and toctree URL handling on notfound_urls_prefix
• add coverage for the Read the Docs 404 title-link behavior

Checks

• venv\\Scripts\\python.exe -m pytest tests\\test_urls.py::test_default_settings tests\\test_urls.py::test_readthedocs_title_links_use_current_version_root tests\\test_urls.py::test_urls_prefix_setting tests\\test_urls.py::test_urls_prefix_setting_none tests\\test_urls.py::test_custom_404_rst_source tests\\test_urls.py::test_special_readthedocs_urls -q
• venv\\Scripts\\python.exe -m compileall -q notfound tests
• git diff --check

Note: a full local tests/test_urls.py run on Windows with Sphinx 8.2.3 had 21 passing tests and one existing failure in test_image_on_404_rst_source around the headerlink character assertion.