Include markdownpart in Release Service for 20.12

[Friedrich W. H. Kossebau]

Hi,

As you might remember, markdownpart is a(nother) Markdown viewer KParts 
plugin, which allows KParts-using software to display files in Markdown format 
in the rendered target format. It mainly is a wrapper around the abilities of 
QTextDocument's Markdown support introduced with Qt 5.14.
Example users of it are Ark, Krusader, Kate's preview plugin & Konqueror.
See it being used with Kate's preview plugin here:
https://cdn.kde.org/screenshots/markdownpart/markdownpart.png

markdownpart passed kdereview this August, and after the small detour into the 
domain of self-managed releases for some quick feedback-based turn-around as 
potentially useful, with though just one release done (0.1.1), I would like to 
ask now for the originally planned inclusion of markdownpart into the Release 
Service starting with 20.12.
It would then be a similar position as the graphics/svgpart project.

See also summary email of kdereview phase in August this year:
https://marc.info/?l=kde-release-team&m=159822183205198&w=2

Once included I would push a respective commit to make use of the 
RELEASE_SERVICE_* version variables.

For reference, the project is located at
https://invent.kde.org/utilities/markdownpart

Cheers
Friedrich