org.kde.kmag.metainfo.xml rename file from org.kde.kmag.appdata.xml
README.md Tokodon -> KMag. src/org.kde.kmag.metainfo.xml :
- Fix "appstreamcli validate org.kde.kmag.metainfo.xml" issue "I: org.kde.kmag.desktop:~: content-rating-missing".
- Use https://apps.kde.org/kmag
- Add URL for app Handbook
(cherry picked from commit 52bd5b78)