Improve stable branch setup/docs

[juliusknorr]

I got reports that creating a new stable branch is not working well and leaving folders in root permissions causing troubles:

• Update failed cannot open /home/julia/docker-nc/server/apps-extra/activity/.git/worktrees/activity/FETCH_HEAD: Permission denied
• Cannot check the working tree for unmerged files because of an error. detected dubious ownership in repository at '/home/julia/docker-nc/stable28/apps/viewer' To add an exception for this directory, call: git config --global --add safe.directory /home/julia/docker-nc/stable28/apps/viewer
• z.B. update_ref failed for ref 'HEAD': cannot update the ref 'refs/heads/stable28': unable to append to '/home/julia/docker-nc/server/apps-extra/text/.git/logs/refs/heads/stable28': Permission denied

Further consider:

• Recommend putting any apps in apps-extra: https://github.com/nextcloud/server/pull/42499#issuecomment-1873813813

[ShGKme]

apps-extra folder is created in the container bootstrap script inside a container, so it has the container's root user as an owner.

https://github.com/juliushaertl/nextcloud-docker-dev/blob/31cbdb61e4d97c4159d55553120df71d914b588c/docker/bin/bootstrap.sh#L371-L372