Change GitHub repository link to Manjaro's GitLab

The current link to the source code in the side navigation points to an nonexistent (or not accessible) GitHub repository.

This merge request just changes the URL to point https://gitlab.manjaro.org/web/manjaro-applications-discovery and change "GitHub" by "Manjaro's GitLab" (this is the better option I thought when I made the change).

Regards, Iván

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