feat: keyboard shortcut to toggle sidebar

[hamirmahal]

closes #2342.

[hamirmahal]

I would have liked to do this in https://github.com/rust-lang/mdBook/blob/master/src/theme/book.js#L443, but for some reason this keyboard shortcut wasn't working there, while the click handler worked without issue.

[hamirmahal]

@Dylan-DPC I tried to do this in a cleaner way with hamirmahal@a7da1d0, but document.addEventListener("keydown", function (event) { wasn't working with that commit, while clicking still toggled the sidebar without issue.

[hamirmahal]

With this change

With this pull request, users would be able to collapse, and expand, the sidebar with just their keyboard.

2025-05-10_15.59.49.mp4

[hamirmahal]

@ehuss are there any changes you'd like me to make here?

I fixed the merge conflicts.

It's been around 414 days or so since I opened this pull request. I'm hoping we can get it across the finish line.

[hamirmahal]

@y21 would you happen to know how I can get feedback from ehuss?

I think this feature could make life easier for a lot of mdBook users. I'm trying to avoid running into merge conflicts again.

[y21]

Sorry, I don't know about the reviewing process in this repo here, usually (at least in clippy and the Rust repo) the standard wait time is 14 days, at which point a reviewer can be rerolled with r? team.
But it looks like this repo only has two maintainers, and it seems there's quite a few other PRs waiting on feedback, too.

This does look like a useful feature though 👍