Always visible table of contents navigation

This is expanding on an idea proposed to Zach on Mastodon

It would be helpful if the side navigation (table of contents) was always visible on the screen, even when scrolling down a long page. The navigation could have its own scroll, so that when large sections are opened (like Guide), the reader could scroll the nav or main body independently, thus keeping the context of the other section. This would be useful when reading about one thing, but wanting to see what related pages there may be. Or when trying to find the right page, skimming down the main body while not changing the position of the nav, therefore allowing you to move to the next page if you don't find what you need.

Additionally, a small amount of JS could be used to scroll the navigation on page load so that the reader's previous position is restored. This will further help them keep context as they move through the docs looking for information. I can see this being done either either heading data or just tracking scroll positions.

No files or tests are named in the issue. Start by locating the documentation site's side-navigation and page-layout entry points, then review how navigation state is currently handled. Done should provide persistent table-of-contents visibility, independent navigation and page scrolling, and restoration of the navigation position when a page loads.