Document how to switch between legacy and new URL of a website

Following https://gitlab.cern.ch/paas-tools/okd4-install/-/merge_requests/266 the change of annotation webservices.cern.ch/webeos-uselegacybaseurl in the UserProvidedDirectory YAML correctly triggers reconciliation (which wasn't the case before).

TODO: add to the "Managing a webEOS site" a section explaining how to change between the legacy and new URL: https://sitename.web.cern.ch/sitename and https://sitename.web.cern.ch

/cc @alossent @dchatzic @kosamara