This adds dynamic navigation of headers of the current page in the
sidebar. This is intended to help the user see what is on the current
page, and to be able to more easily navigate it. The "current" header is
tracked based on the scrolling behavior of the user, and is marked with
a small circle. This includes automatic folding to help keep it from
being too unwieldy on a page with a lot of nested headers.
This includes the `output.html.sidebar-header-nav` option to disable it.
I'm sure there are tweaks, fixes, and improvements that can be made. I'd
like to get this out now, and iterate on it over time to make
improvements.
The reason the ACE editor was failing to load the rust syntax
highlighting is because the syntax highlighting was being created
*after* the editor was created. If the editor is created first, then ACE
tries to load `ace/mode/rust`. Since it isn't already defined, it tried
to compute the URL and load it manually. However, since the URLs now
have a hash in it (via https://github.com/rust-lang/mdBook/pull/1368),
it was unable to load.
The solution here is to make sure `ace/mode/rust` is defined before
creating the editors. Then ACE knows that it can just load the module
directly instead of trying to fetch it from the server.
Fixes https://github.com/rust-lang/mdBook/issues/2700
