Switch to only one top-level heading per doc

Some of our docs have many top-level headings, leading to confusing
toctrees.

To fix this I've found all the top-level headings (except the first,
which is the document title) and made them second-level headings, as
well as demoting all subheadings inside them.

The heading hierarchy is now:

=====
-----
^^^^^
~~~~~

Change-Id: Ie2dbba83d98608e0ad9699c003ac179e67d6da97
Merge "Switch to only one top-level heading per doc"