Minor documentation fixes

Minor fixes to broken links in markdown files. Added a note about
timing adapters being disabled in Arm Corstone-315 by default.

Change-Id: I7f0b33adc8c38e8be7a5bfcebbf5d87f4e22430c
Signed-off-by: Kshitij Sisodia <kshitij.sisodia@arm.com>