Assorted changes: community and l10n content

• Add Krita Artists to community page
• Add links to https://community.kde.org/Get_Involved/translation regarding l10n
• Some capitalization fixes

I don't know what else to write for the Krita Artists section. Please feel free to add to it or revise it.

I also don't know if any of these changes shall be cherry-picked to 5.0.