docs: list SCIP in supported solvers

[FabianHofmann]

Closes #416.

Note

The description below was drafted by AI.

Changes proposed in this Pull Request

SCIP is already supported as a solver backend but was missing from the list of
supported solvers in the docs. This adds it so users can discover it.

• Add SCIP to the supported-solvers list in README.md.
• Add SCIP to the supported-solvers list in doc/index.rst.

Checklist

• AI-generated content is marked (see AGENTS.md).
• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (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 MIT license.

[codspeed-hq]

Merging this PR will not alter performance

✅ 173 untouched benchmarks
⏩ 173 skipped benchmarks¹

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_{Comparing docs/list-scip-solver-416 (9baaf15) with master (065b7ff)}

Footnotes

1. 173 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩