Document UDP versioning & storage guidelines

[jdries]

Expand and clarify guidelines along these lines:

1. UDP's should use git versioning, ensuring stability
2. projects are allowed to store udp json in their own repo
3. changelog is needed

[soxofaan]

UDP's should use git versioning, ensuring stability

FYI there is already some checking/linting in place related to GitHub references: https://github.com/ESA-APEx/apex_algorithms/blob/9cfa13914453097d5bba685aa59da27f31375556/qa/tools/apex_algorithm_qa_tools/common.py#L39-L67
this might have to be updated or aligned with the guidelines we want to enforce