docs-rst: create an user's manual book

Place README, REPORTING-BUGS, SecurityBugs and kernel-parameters
on an user's manual book.

As we'll be numbering the user's manual, remove the manual
numbering from SecurityBugs.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/index.rst

@@ -11,6 +11,7 @@ Contents:
 .. toctree::
    :maxdepth: 2
 
+   admin-guide/index
    kernel-documentation
    process/index
    dev-tools/tools