doc: Limit toc depth to 2 on manpage overview

Doesn't look good if every Arguments list of every subcommand of every
command is listed here for the manpages generated from argparse.

Signed-off-by: Jonas Rebmann <jre@pengutronix.de>

Author: jonrebm

doc/man.rst

@@ -2,6 +2,8 @@ Manual Pages
 ============
 
 .. toctree::
+   :maxdepth: 2
+
    man/client
    man/coordinator
    man/device-config