* feat(forge): add new forge for forgejo
We only support repositories hosted on Forgejo instances, but not
Forgejo Actions or Woodpecker as CI solutions for now.
* test(e2e): introduce e2e test framework with local forgejo
- Renovate does not find the "github:*" dependencies in `mise.toml`
- The `mdbooks` tools was still installed manually with our own action,
this is removed and mise is used instead.
