Improve stable branch setup/docs

[juliushaertl]

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: Update Nc Vue to 8.4.0 nextcloud/server#42499 (comment)

[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.

nextcloud-docker-dev/docker/bin/bootstrap.sh

Lines 371 to 372 in 31cbdb6

	mkdir -p "$WEBROOT/apps-extra"
	mkdir -p "$WEBROOT/apps-shared"