[docs] Add global table of contents

[oliver-ni]

Summary

Addresses #9613.

This PR adds the Sphinx global TOC (site navigation) on the left side of the screen, moving the local TOC (table of contents for the current page) to the right. Of course, it also works on mobile.

This is a fairly low-effort implementation in that I tried to simply work around the CSS that was already there instead of making a lot of big changes. At a high level, I split the styles for aside into #local-toc and #global-toc, and since we now have multiple TOCs displayed, renamed #sidebar to .toc. There is also a bit of duplicate CSS with this since the two TOCs collapse at different breakpoints.

There seems to be some amount of cruft with the workings of the templates/styles in general, and I'd be happy to do a little bit of a cleanup, or a bigger rework, depending on what's wanted. I also just need to make the global TOC look a little nicer. Regardless, I'm pushing these changes for now so people can have a look and give thoughts.

Preview: https://oliver.ni/dpy-docs-preview/

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)