Replace signature nodes with sections

This is an attempt to provide a setup that does not indent the contents
of the 'Public Functions' parts of the documents. Using a
desc_signature node for that part leads to the contents being indented
which isn't necessary and adds to the already significant amount of
indentation that we have.

Here we replace the signature and contents with an official 'section'
node which doesn't do any indentation.
Preserve document structure

Replace section with container and title with rubric. According to
docutils. This preserves the document structure but gives the required
rendering of titles like "Public Functions".
Add breathe class names to section & rubric

So that we have something more specific to style with on the html side
if we're keen to.
Update changelog for reformatting changes
Merge branch 'sections'