This commit makes three changes for consistency and readability:
- It shows the sibling navigation on module pages. It's weird
that it didn't work before, and is inconsistent with everything
else (even Crates have sibling navigation with other Crates).
- It hides the "In [parent]" header if it's the same as the
current crate, and if there's no other header between them.
We need to keep it on modules and types, since they have
their own header and data between them, and we don't want
to show siblings under a header implying that they're children.
- It adds a margin to deal with the headers butting directly into
the branding lockup.
47c46324aa