gh-125895: Docs: fix 404 page prefix configuration

[m-aciek]

The default is /en/latest/. Our docs use /3/ for latest stable version documentation build. That way links and resources URLs will be correct in the generated 404 page, in the docs.python.org subdomain.

Closes #125895 if followed-up by nginx's (https://nginx.org/en/docs/http/ngx_http_core_module.html#error_page) directive config.

(This change doesn't require a news entry.)

• Issue: Docs: sphinx-notfound-page doesn't show nicer 404 page #125895

---

📚 Documentation preview 📚: https://cpython-previews--139386.org.readthedocs.build/

[m-aciek]

Doesn't break the RTD configuration: 404 in preview is still rendered correctly.

[hugovk]

Will this work for other version builds such as https://docs.python.org/3.15/ and https://docs.python.org/dev/ ?

And for translations such as https://docs.python.org/pl/3/ and https://docs.python.org/pl/3.15/ ?

[AA-Turner]

Indeed, this won't work properly. We would need to do this dynamically in docsbuild-scripts.

As such, I'll close.

A

[m-aciek]

We should set https://docs.python.org/3/404.html as a global 404 page (for whole subdomain). That way it will work for all versions.

Nice to have would be to set a 404 page per build, but:

• that would (superfluously?) complicate the nginx configuration, and
• without improved notfound_urls_prefix to support /{language}/{version}/ dynamically, we'd incorrectly link to current stable saying it's a version from build:

<li id="cpython-language-and-version">
  <a href="/3/index.html">3.15.0a0 Documentation</a> &#187;
</li>

• we'd load JS and styles from current stable version rendered with Sphinx 8.2.3; that potentially could cause incompatibilities, when 404 was rendered with an old Sphinx version (e.g. Sphinx 2.4 from Python 3.9); for example:

    <script src="/3/_static/documentation_options.js?v=048d8b21"></script>
    <script src="/3/_static/doctools.js?v=9bcbadda"></script>
    <script src="/3/_static/sphinx_highlight.js?v=dc90522c"></script>
    
    <script src="/3/_static/sidebar.js"></script>

I think having a global 404 with English latest stable link (without the nice to have) is just fine, or at least fine until we figure out how to improve it.

[m-aciek]

It can be tested locally e.g. this way:

cpython % cd Doc
Doc % make html
…
Doc % cd build
build % mv html 3  # to imitate directories layout on the production server
build % python -m http.server  # browse to http://localhost:8000/3/404.html