doc: Normalise section numbering and headings

Required work to make all documents sit at the correct levels within
the document tree and any derived content like the table of contents
and the categories in the sidebar.

Change-Id: I4885fbe30864a87c8822ee67482b71fb46a8fbc6
Signed-off-by: Paul Beesley <paul.beesley@arm.com>
6 files changed