Add AlphaEvolve benchmark documentation

[nikivanstein]

Motivation

• Add documentation surface for the AlphaEvolve benchmark instances used in the project so users can discover and run them easily.
• Surface both the runnable reference scripts and the programmatic benchmark definitions in the project README and Sphinx docs.

Description

• Added an AlphaEvolve Benchmarks section to README.md that links the runnable scripts in run_benchmarks/ and the packaged benchmark definitions under iohblade/benchmarks.
• Added a new Sphinx page docs/benchmarks.rst that summarizes the benchmark domains and points users to task READMEs and sources.
• Updated docs/index.rst to include the new benchmarks page in the Sphinx toctree so the page appears in the documentation.

Testing

• No automated tests or linters were run because this change only modifies documentation files and adds a new docs page.

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Codex Task