Some external links to documentation are broken

[manics]

Bug description

#1433 reorganises the repo2docker documentation.
This has resulted in some broken links in external projects that reference repo2docker, e.g.
https://repo2docker.readthedocs.io/en/latest/config_files.html
https://github.com/jupyterhub/zero-to-jupyterhub-k8s/blob/151bd281e71dfac9d4c4a2930e4adbc635b608be/docs/source/repo2docker.md?plain=1#L33

Expected behaviour

Old pages should either redirect to the new location

Actual behaviour

Some existing URLs lead to a 404

[agitter]

#1466 fixed some of these in the readme