Allow overwriting docs URL; select correct default config in test

[brynpickering]

Noticed the test failing on using #2056 in anger on a soft fork + my custom default config being littered with references to the pypsa-eur docs.

This PR handles the selection of the correct project-level default config in the test + allows the user to update the docs ref used in that default config file to point to a different URL, if desired.

Checklist

• I tested my contribution locally and it works as intended.
• Code and workflow changes are sufficiently documented.
• Changed dependencies are added to pixi.toml (using pixi add <dependency-name>).
• Changes in configuration options are added in config/config.default.yaml.
• Changes in configuration options are documented in doc/configtables/*.csv.
• For new data sources or versions, these instructions have been followed.
• A release note doc/release_notes.rst is added.