Update sphinx theme

[dannycolin]

Details

Use the default readthedocs theme instead of our custom theme. This ensure we use the same theme for both 5.x and 6.x documentation. We also benefit from niceties included in the latest readthedocs theme that our custom theme doesn't provide (e.g better mobile layout).

Additional info

bug 2010419

NOTE: Bug number is required. If you haven't filed a bug on https://bugzilla.mozilla.org/enter_bug.cgi?product=Bugzilla yet, don't submit the Pull Request. Please edit the above link to put the bug number both immediately after the # character and after the = character at the end. You can delete this paragraph once you've added your bug number.

Test Plan

cd [harmory folder]/docs/en
python -m venv venv
pip install -r requirements.txt
make html
python -m http.server -d ./html

You might need to adjust the sphinx version installed in your venv depending on your OS (e.g. Fedora 43 requires Sphinx 7.3+ because it comes with Python 3.14). This shouldn't be an issue in the runner.

[justdave]

Using the GitHub link to rebase this branch since it's out of date...

[justdave]

Build of this is available at https://bugzilla--152.org.readthedocs.build/projects/harmony/en/152/