Upgrade Sphinx Theme to 1.12 and remove template overrides and CSS files

[Eric-Arellano]

Did you add tests to cover your changes (yes/no)?

n/a.

Did you update the documentation accordingly (yes/no)?

n/a

Did you read the CONTRIBUTING document (yes/no)?

yes

Summary

Upgrades to qiskit-sphinx-theme 1.12. See https://github.com/Qiskit/qiskit_sphinx_theme/releases/tag/1.11.0 and https://github.com/Qiskit/qiskit_sphinx_theme/releases/tag/1.12.0rc1 for the detailed changelogs.

Some highlights:

• Fixed IBM fonts not loading
• Left sidebar can have expandable sections
• Page table of contents on the right stays fixed when scrolling

This makes some other docs improvements:

• Removes layout.html and page.html template overrides. Those should come from qiskit-sphinx-theme.
• Activates jQuery, since it is no longer activated by default in Sphinx 6+. It is necessary for qiskit-sphinx-theme.
• Simplifies how Tox installs the package to align with all other ecosystem projects
• Moves sphinx-intl to requirements-dev.txt and pins it.

Finally, this removes some unnecessary CSS files:

• gallery.css is for sphinx-gallery, which is not used by this project.
• style.css is leftover from when we used the Read The Docs theme. It's no longer relevant to qiskit_sphinx_theme.
• custom.css only has rules for .toggle, which does not exist anymore.

[zlatko-minev]

Thanks, @Eric-Arellano, Re "n/a. But note that I couldn't build the docs locally because I have an M1 and pyside2 cannot be built on Apple Silicon." I wonder if we should add a simple workaround in the spot that you see it fail

I think i understand this worked ok for you and the checks have passed, Going to merge in after updating and the update checks complete

[Eric-Arellano]

Thanks, @zlatko-minev! I couldn't get Linux working on my M1, but I realized this morning that you already build the docs in CI and you can download the artifact to open up the docs locally 🎉 I inspected the docs that CI built and they all look good. This PR is also very similar to ~15 other PRs we've merged in the rest of the ecosystem.

This is ready to merge.