Extend documentation of forwarding functionality

[bernhardreiter]

Split out of #384:

• add some more documentation about the forwarding functionality, especially link to where the protocol is documented.
• potentially add runtime test(s)

[s-l-teichmann]

See https://github.com/mfd2007/csaf_upload_interface/issues/13 Doku like https://github.com/mfd2007/csaf_upload_interface/tree/main/tests ?

[bernhardreiter]

As mfd2007/csaf_upload_interface#13 is closed now, with the api.yaml being corrected, the implementation now matches the apt.yaml.

It is also acceptable to link to the documentation in https://github.com/mfd2007/csaf_upload_interface as long as there is no other documentation of an implementation of a document receiving application that is leading. So we need to add an appropriate link to our documentation.