docs: add Liander 2024 benchmark results page under Guides

[egordm]

Summary

Adds a Benchmark Results guide that documents how each OpenSTEF model performs on the public Liander 2024 benchmark, so users have a concrete, data-driven reference for model selection.

What's included

• At-a-glance grouped box-plot of rCRPS by target group (docs/source/images/guides/liander2024_rcrps_by_group.png).
• The Metrics section explaining rCRPS and the sample-weighted variant (with KaTeX formulas).
• Two results tables (mean rCRPS across targets) — unweighted and sample-weighted — with rows per model (XGBoost, GBLinear, Ensemble) and columns per target group (Global, MV feeder, Station inst., Transformer, Solar park, Wind park).
• How these numbers were produced methodology note. The ensemble is an openstef-meta learned-weight combination of LightGBM and GBLinear base models.

Wiring

• Linked from the Guides index as a new card + toctree entry.
• Cross-linked from the Model Selection concept page via a seealso.

Notes

• REUSE-compliant: new PNG has a .license sidecar; the .rst carries an SPDX header.
• Docs build passes locally (sphinx-build EXIT=0; only pre-existing autodoc warnings remain).

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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