Building my own UCS

I try to build UCS from the git sources and I am starting with “doc”.
Now I am running into the first Problem at univention-corporate-server source in the folder doc. The folder doc-common is missing if I clone the repository.

Is this a bug? Do i miss something?

I still try to build the UCS-documentation - from time to time - now on the 5.0-2 branch.
I try to follow the instructions in README.txt in the doc/achitecture folder.
Maybe my open problems relay on this information in the README.txt:

The KNUT CA certificate must be part of your local certificate installation.
  See `Install KNUT domain root CA certificate in the Dev Onboarding

The link above is not working for me. So I might miss a certificate while executing

pip install -r requirements.txt --cert /etc/ssl/certs/ca-certificates.crt

Can somebody help me?