Is your feature request related to a problem? Please describe.
Not quite a problem, I can use via https://nbviewer.org/urls/, but I cannot pass a link to dir this way.

Describe the solution you'd like
The same level of support as GitHub has. There should be a generic gogs provider to pass a link to any Gogs/Gitea (a fork of Gogs)/Forgejo (a fork of Gitea) instance. And a codeberg to pass a link to Codeberg well-known Forgejo instance. And IMHO the l8st of well-known services (like github) should be configured in a config file, not via an env variable.

Describe alternatives you've considered
No.