The Picard documentation is being moved from the GitHub Pages server to ReadTheDocs.  The `https://picard-docs.musicbrainz.org` URL will need to be redirected to `https://picard-docs.readthedocs.io`.  Once this has been changed, the Picard 3 constant `DOCS_SERVER_URL` can be changed back to "https://picard-docs.musicbrainz.org/" from the temporary setting "https://picard-docs.readthedocs.io/".  The `gh-pages` branch from the `picard-docs` repository can also be removed.