doc: Link coding guidelines to user guide

Adds a link from user-guide.rst to coding-guidelines.rst and merges
the information about using checkpatch from both files into the user
guide document.

Change-Id: Iffbb4225836a042d20024faf28b8bdd6b2c4043e
Signed-off-by: Paul Beesley <paul.beesley@arm.com>
2 files changed