[documentation] update links in the readme to reference the public repository

Unfortunately there's no great way right now to have separate versions of the README for the public and private repositories, so the best thing to do for visibility's purpose is to have all links in the README reference the public copy. The developer docs, however, will still give instructions for developing on the private repo.

This should not be merged until I've ensured that the pipelines are running and that the docs have been built on GitLab pages on the public repo.