rtla build fails due to doc build dependency on rst2man. Make
doc build optional so rtla could be built without docs. Leave
the install dependency on doc_install alone.

Link: https://lkml.kernel.org/r/20220126001301.79096-1-skhan@linuxfoundation.orgAcked-by: NDaniel Bristot de Oliveira <bristot@kernel.org>
Signed-off-by: NShuah Khan <skhan@linuxfoundation.org>
Signed-off-by: NSteven Rostedt (Google) <rostedt@goodmis.org>