Feature request: support translating the generated HTML files

Is your feature request related to a problem? Please describe.
When adding full translation to a project's documentation, I want to be able to
fully translate the documentation, even the generated URIs.

Right now Sphinx supports everything by using gettext, image filename specific
to a language, etc. This allows me to translate the images, content, headers,
except the generated link.

Say I have a file called contributing.rst. When translating it's title to
pt_BR, I can add the "Contribuindo" translation in the generated .po file as
the translationg to the original "Contributing" title, but the final URI still
will be example.com/pt/contributing.html. Even anchors in the existing page
are translated, which is great, only the filename itself that isn't.

Describe the solution you'd like
Be able to have sphinx-doc understand such translation option. No real clue on
any solution on this, though.

Describe alternatives you've considered
I've hacked a brute force solution, by:

1. adding a i18n-pagenames.json file and adding filename translations to it;
2. replacing occurences of {{ pagename }} in my templates to {{ i18n_pagenames[lang][pagename] }} to link to the translated pagename;
3. renaming files in the temporary build directory with mv $from.rst $to.rst;
4. running sed -i "/^ $from/ $to/" index.rst for each entry in the
   i18n-pagenames.json so that the table of contants include the translated
   version.

By doing this fake replacement before running sphinx-build, everything works
fine.

I'm happy with my current solution, but I was wondering if this is something
that should be added to Sphinx itself.

I might be able to help implement it with some guidance if possible, but I'm not
in any hurry.

Additional context

Commit where I implement the brute force approach.

Start by tracing sphinx-build's gettext flow from the generated .po files through HTML URI and anchor generation. Compare that behavior with the issue's i18n-pagenames.json, template {{ pagename }} replacement, temporary-file renaming, and index.rst workaround. Done should mean translated page filenames and links are handled by Sphinx without those external substitutions.