Add a cookbook landing page indexing all examples and recipes

[maziyarpanahi]

Description

Adds a task-oriented Cookbook landing page that maps common OpenMed goals to the exact existing scripts, notebooks, and copy-ready recipes that implement them. This makes the repository's current examples discoverable without replacing the longer examples guide.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring
• Performance improvement
• Test addition/improvement

Changes Made

• Add a task-first Cookbook covering the in-scope PII, clinical NER, tokenizer, service, privacy-gateway, pipeline, and notebook assets.
• Add source-link validation so every indexed repository example resolves to an existing file.
• Add the Cookbook to the documentation navigation and cross-link it from the existing examples page.

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested this change with different models/inputs

Validation completed:

• .venv/bin/python -m pytest tests/ -q — 5,201 passed, 46 skipped
• .venv/bin/python -m mkdocs build --strict
• pre-commit run --files docs/cookbook.md docs/examples.md mkdocs.yml tests/unit/test_docs_cookbook_links.py

Documentation

• I have updated the documentation accordingly
• I have added docstrings to new functions/classes
• I have updated the CHANGELOG.md

No public Python API or release metadata changed.

Code Quality

• I ran make format, make lint, and make format-check
• For Swift/OpenMedKit changes, I ran make format-swift and make lint-swift
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings

Dependencies

• I have not added any new dependencies
• OR I have added new dependencies and they are justified because: ____

Checklist

• I have read the contributing guidelines
• My commits have clear, descriptive messages
• I have squashed/organized my commits appropriately

Related Issues

Closes #478

Screenshots/Examples

Not applicable; this change adds documentation and validation coverage.