Docs: Move design documents from wiki to source

Moving design documents from Phabricator
(https://developer.trustedfirmware.org/w/tf_m/design/

) to the source
tree.

The documents moved in this commit are:
 - Code Generation With Jinja2
 - Non-Secure Interrupt Handling
 - Non-Secure Client Management
 - Secure Partition Interrupt Handling
 - Uniform Secure Service Signature

Change-Id: I442f1c63633f493e3f0d1b9ffc42211f584a6ecc
Signed-off-by: Mate Toth-Pal <mate.toth-pal@arm.com>