This switches to just use size, weight, and spacing to distinguish
headings in the sidebar. We no longer use boxes, horizontal bars, or
centering to distinguish headings. This makes it much easier to
understand the hierarchy of headings, and reduces visual noise.
I also refactored how the mobile topbar works. Previously, we tried to
shift around elements from the sidebar to make the topbar. Now, the
topbar gets its own elements, which can be styled on their own. This
makes styling and reasoning about those elements simpler.
Because the heading font sizes are bigger, increase the sidebar width
slightly.
As a very minor change, removed version from the "All types" page. It's
now only on the crate page.
This is a fairly common attribute on methods, but is not something you
need to know when reading the method docs - the purpose of the attribute
is for the compiler to tell you about it if you forget to use a value.
Removing reclaims some valuable space in the summary of methods.
