Add new codespace link to docs

Right now https://github.com/nextcloud/server/blob/master/.devcontainer/README.md just points to the docs of codespaces.

However, it would be nice to add the following link to directly spin up a new instance:
https://github.com/codespaces/new?hide_repo_select=true&ref=master&repo=60243197&skip_quickstart=true

Edit: also would be nice to note that it takes 5+ minutes to build