update source and description of "electricity grid connection"

[lindnemi]

Closes # (if applicable).

Changes proposed in this Pull Request

Added the data source for "electricity grid connection" and explained that it corresponds to grid expansion costs

Actually, this entry is named a bit unfortunately. It should be called "distribution grid expansion for new assets" or something like that, because the actual cost of grid connection is already included in the PV / Onwind costs per kw.

I didn't want to brake downstream models, so i only updated the description.

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Data source for new technologies is clearly stated.
• Newly introduced dependencies are added to environment.yaml (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the GPLv3 license.