Adjust IMA-templates.txt for ReST markup and add to the index for
security/, under the Kernel API Documentation.

Cc: Mimi Zohar <zohar@linux.vnet.ibm.com>
Signed-off-by: NKees Cook <keescook@chromium.org>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>