doc: Update user guide
This commit updates the user guide to bring it in-line with the latest v2.5.0 release. It changes the Markdown header syntax in order to represent deeper header levels, replaces old links and attempts to clear up areas of potential confusion with instructions.
Change-Id: I99b464a4bc94f4618779fbe09b1aa77bed0c318e Signed-off-by: Chris Kay chris.kay@arm.com