Collapse sections of the sidebar by default

[j-f1]

The first time I visited the Mastodon docs, I felt intimidated/confused by the amount of information in the sidebar. This PR helps remedy that by collapsing all of the sections when JS is enabled:

Additionally:

• The section containing the current page is always expanded
• There’s an “expand all” button to easily expand all of the sections so it’s just 1 additional click to search through all of the page titles. (There is something Chrome supports called hidden="until-found" so you could just ctrl+f and find everything hidden but I don’t use Chrome so I don’t know how to make it work)

[ThisIsMissEm]

@j-f1 would it be possible for you to rebase this branch? If not, I could do it and create a PR to supersede this one (giving you credit of course)

[vercel]

@j-f1 is attempting to deploy a commit to the Mastodon Team on Vercel.

A member of the Team first needs to authorize it.

[j-f1]

Rebased & moved to the main.js file.

[github-actions]

This pull request has merge conflicts that must be resolved before it can be merged.