Documentation/admin-guide: update admin-guide index.rst

Extend index.rst index file at admin-guide root directory with
the reference to perf-security.rst file being introduced.
Signed-off-by: Alexey Budankov <alexey.budankov@linux.intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/index.rst

@@ -76,6 +76,7 @@ configure specific aspects of kernel behavior to your liking.
    thunderbolt
    LSM/index
    mm/index
+   perf-security
 
 .. only::  subproject and html