Add Ghana (GH) country configuration to PyPSA-Earth

[kwaku-duah]

Summary

This PR adds Ghana as a supported country in PyPSA-Earth with optimized configuration settings.

Changes

• Set countries to ['GH']
• Adjust voltage threshold to 33kV for Ghana's grid
• Configure map boundaries for Ghana visualization
• Keep offshore wind carriers (Ghana has coastline)

Related Work

Full Ghana energy modeling implementation available at: https://github.com/kwaku-duah/pypsa-gh/tree/ghana-model

This PR contributes the official country configuration to PyPSA-Earth, while detailed tutorials, validation notebooks, and decarbonization case studies are maintained in the separate repository above.

• Set countries to ['GH']
• Adjust voltage threshold to 33kV for Ghana's grid
• Configure map boundaries for Ghana visualization
• Keep offshore wind carriers (Ghana has coastline)

Closes # (if applicable)

Changes proposed in this Pull Request

Checklist

• I consent to the release of this PR's code under the AGPLv3 license and non-code contributions under CC0-1.0 and CC-BY-4.0.
• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented, including updates to docstrings for meaningful functions.
• Newly introduced dependencies are added to envs/environment.yaml and doc/requirements.txt.
• Changes in configuration options are added in all of config.default.yaml and config.tutorial.yaml.
• Add a test config or line additions to test/ (note tests are changing the config.tutorial.yaml)
• Changes in configuration options are also documented in doc/configtables/*.csv and line references are adjusted in doc/user-guide/configuration.md and doc/tutorials/electricity-model.md.
• If config sections were added, renamed, or removed, update doc/assets/scripts/extract_config_snippets.py accordingly.
• Archives of the uploaded data do not have an enclosing folder and archive names correspond to the conventions of configs/bundle_config.yaml.
• A note for the release notes doc/release-notes.md is amended in the format of previous release notes, including reference to the requested PR.

[kwaku-duah]

@pypsa-meets-earth/developers @ekatef

Could you please review this Ghana configuration contribution?
The config is minimal and has been tested locally. I want to a contribution, while I keep working on a separate branch on my fork to allow lots of other contributors to work on Ghana's modelling. All feedbacks are welcome, for improvements, something not done well etc