doc: ReSTify IMA-templates.txt

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: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/security/index.rst

@@ -1,7 +1,9 @@
 ======================
-Security documentation
+Security Documentation
 ======================
 
 .. toctree::
+   :maxdepth: 1
 
+   IMA-templates
    tpm/index