Migrate documentation
The MR moves from the previous mkdocs documentation scheme to the new one: https://how-to.docs.cern.ch/gitlabpagessite/migration/
The website will be served via these links:
- https://umami.docs.cern.ch (new link)
- https://umami-docs.web.cern.ch (same as before for continuity)
The reason for this change is that the web service hosting the umami documentation has changed.