doc: Fix incorrect component names and heading structure

Replace all instances of 'the RSE' with RSE where the
docs are purely referring to RSE.
Trusted Firmware always refers to RSE without a determiner.

Update headings to all be sentence case
A review by the tech writers group a while back commented that
we should be using sentence case for all our headings. This
commit addresses that comment.

Fix any instances where component names should be all first
letter capitalized and aren't.
E.g. Safety island instead of Safety Island.

Signed-off-by: Anna Bridge <anna.bridge@arm.com>