Make gitbay the canonical forge in code and docs !20

Stacked on !19; retarget to main once that merges.

Origin, issues and merge requests are on gitbay, so the code and docs now say so: RefreshInstances fetches instances.json from https://gitbay.org/krz/skunky-art/raw/main/, the About page's "Report an issue" and release links, the index source link, --help and the --add-instance message all point at gitbay, and INSTANCES.md links the gitbay raw file. The GitHub mirror stays for the container image build; the README has a section saying which is which. Dependabot and the release workflow are untouched since they only make sense on the mirror.

Closes #31