Changes proposed in this Pull Request

In NEP links, only the "Zubaunetz" (extension network) was considered. With the new links, also the "Startnetz" (start network) is considered.
Image of links in Germany:

Checklist

• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• Changed dependencies are added to envs/environment.yaml.
• Changes in configuration options are added in all of config.default.yaml.
• Changes in configuration options are also documented in doc/configtables/*.csv.
• A release note doc/release_notes.rst is added.