docs: add methodology taxonomy and annotate benchmark entries

[nikivanstein]

Motivation

• Provide a clear taxonomy that groups packaged benchmarks by optimization methodology for better discoverability and classification.

Description

• Add a new "Methodology Taxonomy" section to docs/benchmarks.rst that defines four methodology classes: Black-box Continuous, Black-box Discrete/Mixed-Integer, Structured Scientific Program/Search, and ML Pipeline Optimization.
• Annotate existing benchmark families and individual benchmark entries with their corresponding methodology class references throughout docs/benchmarks.rst.
• Add mabbob to the BBOB family mention and adjust several section headers and cross-references to point to the new taxonomy.

Testing

• Built the documentation with Sphinx using make -C docs html to verify the reStructuredText compiles and cross-references resolve, and the build completed successfully.

---

Codex Task

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!