docs: Revamp Plotting & Summary Visualization Documentation

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Summary

The previous Plotting documentation section was removed during the MkDocs migration (PR #1638) as it was not particularly meaningful in its current state — it mainly contained an outdated color palette image with limited practical guidance.

However, plotting and result visualization are an important part of the PyPSA-Earth workflow and deserve proper documentation.

Scope

Create a new, comprehensive Plotting & Visualization documentation page that covers:

• Summary scripts — Document how make_summary.py and plot_summary.py work and what outputs they produce
• Result visualization — How to visualize network results (generation mix, line loading, etc.)
• Color palettes — Technology color schemes used in PyPSA-Earth plots (kept up-to-date with the actual config)
• Jupyter notebook examples — Link to or embed example plotting notebooks
• Custom plotting — Tips for users who want to create their own visualizations from PyPSA outputs

Context

• Removed in PR #1638 (MkDocs migration)
• Original review comment by @ekatef: "Currently, the section is not particularly meaningful, except the palette picture which could get outdated. While the plotting and summary is certainly an important part, I wonder if it could be a sensible solution to drop this section for now adding an issue on tackling this part?"

Acceptance Criteria

• A new doc/user-guide/plotting.md page with practical, up-to-date content
• Corresponding entry added back to mkdocs.yml navigation under User Guide
• At least one working example or screenshot of typical plot outputs