Add example for hosted git service issue autoexpansion

[HarHarLinks]

• supports multiple git cloud services with examples for GitHub, GitLab (incl hosted instances), Codeberg (Gitea/Forgejo) with their different URL schemes
• supports default projects and orgs (i.e. understands #16, reactbot#16, or maubot/reactbot#16)
• aliases for long project names (e.g. hookshot#123 -> matrix-org/matrix-hookshot#123)

I would prefer to simplify the templates into 1 by #16, but it might be possible to simplify the templates into a single one with jinja.