Currently, sidebar TOCs only assume one level of nesting, and don't obey
excludes. This leads to both missing items and unexpected items that should
have been excluded.

* Rewrite it to be recursive, such that all subsections are generated and
  appended as children.
* Also obey excludes and don't add anchors for them.

There was also a fixture for nested sections, but no corresponding test. Used
that fixture to test this out. (I've also verified that it works against
nextest's site.)