Fix checkupdates and schema validation in quick start guide

It seems I need a special flag when I run fdroid checkupdates before git commit to determine the latest version, rather than afterwards to check it.

Also I expect it to add the line with the most recent CurrentVersionCode automatically if missing, but it throws an error: "checkupdate failed... int() argument must be a string, a bytes-like object or a number, not 'NoneType'".

Duplicating the Repo value in the SourceCode field seems redundant; the fdroid rewritemeta command does not add it and the fdroid lint command does not complain offline about it missing, but the merge request pipeline fails unless I add it.