Add license and license of documentation to user manual

[tfr42]

Currently the user document is missing a statement of the license under which the source code of deegree OGC API is published and also the license of the documentation itself.
The proposed licenses for the documentation are

• GNU Free Documentation License
  or
• CC BY

Same license should be applied to https://github.com/deegree/deegree3/blob/main/deegree-services/deegree-webservices-handbook which is referenced.