Fix Bug 285162: adjust documentation view font settings by values taken from system settings (DE)

Summary:
The patch sets Sans Serif font as default for documentation view.
Font settings are adjusted to system setting (DE):

* Sans Serif font (we assume that is the default system font).
* Monospace font.
* Sizes for all fonts.

Only Serif font family setting is taken from fontconfig (system settings for such font is missing).

BUG: 285162

Test Plan:
Tested with master branch.

{F346398}

Reviewers: #kdevelop, apol, brauch

Reviewed By: #kdevelop, apol, brauch

Subscribers: brauch, cordlandwehr, apol, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D2902