Documentation/admin-guide: introduce perf-security.rst file

Implement initial version of perf-security.rst documentation file
covering security concerns of perf_event_paranoid settings.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Alexey Budankov <alexey.budankov@linux.intel.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>