Add bug-reference support for Codeberg projects

* lisp/progmodes/bug-reference.el (bug-reference-setup-from-vc-alist):
Add support for codeberg.org bug and pull request references.
* doc/emacs/maintaining.texi (Bug Reference): Mention that bug and
pull request references for codeberg projects are supported.
3 jobs for master in 52 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Fast
failed #23410
test-fast-inotify

00:00:31

 
  Normal
failed #23411
test-lisp-inotify

00:00:19

failed #23412
test-lisp-net-inotify

00:00:20

 
Name Stage Failure
failed
test-lisp-net-inotify Normal There has been a runner system failure, please try again
Fetching changes with git depth set to 50...
Reinitialized existing Git repository in /builds/emacs/emacs/.git/
From https://emba.gnu.org/emacs/emacs
* [new ref] f770538b30e5b55905617f293eb933b9d6e93ec1 -> refs/pipelines/11426
ec3b108..f770538 master -> origin/master
Checking out f770538b as master...

Skipping Git submodules setup
ERROR: Job failed (system failure): Error response from daemon: Conflict. The container name "/runner-2oEDrUT7-project-1-concurrent-2-build-4" is already in use by container "da1fe579c662349f5b0cbedeaf385ca8567e208eca419e08e6d3defd24c4f2dc". You have to remove (or rename) that container to be able to reuse that name. (executor_docker.go:726:0s)
failed
test-lisp-inotify Normal There has been a runner system failure, please try again
Using docker image sha256:81f5749c9058a7284e6acd8e126f2b882765a17b9ead14422b51cde1a110b85c for docker:19.03.12 ...
Running on runner-2oEDrUT7-project-1-concurrent-1 via emba.gnu.org...
$ git config --global http.proxy $HTTP_PROXY; git config --global https.proxy $HTTPS_PROXY
Fetching changes with git depth set to 50...
Reinitialized existing Git repository in /builds/emacs/emacs/.git/
Checking out f770538b as master...

Skipping Git submodules setup
ERROR: Job failed (system failure): Error response from daemon: Conflict. The container name "/runner-2oEDrUT7-project-1-concurrent-1-build-4" is already in use by container "f46df44a43bee1f61a6ede2eac732a4e06eaa2094659f7ea23718f093d67bcc6". You have to remove (or rename) that container to be able to reuse that name. (executor_docker.go:726:0s)
failed
test-fast-inotify Fast There has been a runner system failure, please try again
Fetching changes with git depth set to 50...
Reinitialized existing Git repository in /builds/emacs/emacs/.git/
From https://emba.gnu.org/emacs/emacs
* [new ref] f770538b30e5b55905617f293eb933b9d6e93ec1 -> refs/pipelines/11426
ec3b108..f770538 master -> origin/master
Checking out f770538b as master...

Skipping Git submodules setup
ERROR: Job failed (system failure): Error response from daemon: Conflict. The container name "/runner-2oEDrUT7-project-1-concurrent-1-build-4" is already in use by container "f46df44a43bee1f61a6ede2eac732a4e06eaa2094659f7ea23718f093d67bcc6". You have to remove (or rename) that container to be able to reuse that name. (executor_docker.go:726:0s)