Enable sidebar navigation folding

[ehuss]

This enables the sidebar navigation folding feature in mdbook (https://rust-lang.github.io/mdBook/format/configuration/renderers.html#outputhtmlfold) which collapses the sidebar by default. You can look at https://rustc-dev-guide.rust-lang.org/ as an example of folded chapters.

The main issue is that the sidebar has become quite long over time, and it can make it a little harder to see the big picture. Particularly the triagebot and infrastructure sections have a large number of sub-chapters which can overwhelm the navigation.

I don't love this 100% because it can make it a little harder for certain navigation use-cases. For example, I can't use my browser's search functionality and enter "zulip" to find the zulip-related chapters. I now have to use mdbook's search bar, and mdbook's search output isn't particularly great (especially the ranking). But overall I think this is a net win.

r? @rust-lang/internal-sites

[Urgau]

I agree that triagebot's sub-chapters take too much space. Collapsing from the top-chapter seems fine.

As an extension to folding the chapters, we can probably move some chapters around to make them more discoverable.

[apiraino]

Noting that the chapter folding does not seem to work without javascript enabled.

(I don't mean to diss on this proposal, I guess I'm just in the minority when testing these cases)

[jieyouxu]

Noting that the chapter folding does not seem to work without javascript enabled.

(I don't mean to diss on this proposal, I guess I'm just in the minority when testing these cases)

No that's appreciated, especially if there's an much bigger issue of not just "folding don't work" but "all the subchapters get hidden and you can't unhide them unless you have js enabled".

[jieyouxu]

I do agree that the current chapters are getting a bit unwieldy, so I would be both in favor of:

1. Trying out the folding in this PR. At worst, we can revert.
2. We can also do some reorganization, as Urgau mentioned.

[jieyouxu]

Thanks