docs(threat-model): cosmetic changes

 - Add empty lines after titles.

 - Reduce number of highlighting characters to fit title length.

 - Remove most ``monospaced text``.
   I think most of it looked weird in the rendered HTML version and
   it had no obvious meaning.

Change-Id: I5f746a3de035d8ac59eec0af491c187bfe86dad7
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>