Summary:
rst2html reports warnings about the rst syntax into the generated html,
unless told to switch to another report level, like with --quiet.
As the user cannot (or does not want to) do anything about the cmake docs
when just using them, hide any warnings from them.

Test Plan:
CMake docs are still shown, now without any rst syntax warnings (e.g. for
CMake's "project".

Reviewers: #kdevelop, apol, kfunk

Reviewed By: #kdevelop, kfunk

Subscribers: kdevelop-devel

Tags: #kdevelop

Differential Revision: https://phabricator.kde.org/D15919