Add derives reference page and overview links Closes #34

[binaek]

Summary

• Rewrites the derives reference page to match runtime and index behavior from issue #88.
• Removes incorrect guidance that TypeScript modules may be called from derive bodies.
• Documents purity rules, policy scope, HOF callbacks, and export constraints.

What this PR does

• Corrects derives.md so derive purity explicitly forbids TypeScript/use module calls.
• Adds visibility rules for namespace vs policy-scoped derives and cross-policy FQN boundaries.
• Documents single-parameter derive callbacks for collection builtins.
• Cross-links lambdas and built-in functions pages to the derive callback section.

Changes By Area

Reference documentation

• src/content/docs/reference/derives.md: Full rewrite — syntax, calling, scope, purity whitelist, callbacks, exports, examples, corrected See also links.
• src/content/docs/reference/index.md: Lambdas section notes derive callbacks for HOF builtins.
• src/content/docs/reference/built-in-functions.md: List builtins section notes derive-by-name callbacks.

Review Notes

• Verify prose matches sentrie branch 88-add-derive-for-named-pure-expressions-using-typed-lambda-syntax behavior (no TS in derives, policy scope, export rules).
• Confirm examples use valid Sentrie syntax and three-segment slash FQNs where shown.

Testing Notes

• Docs-only change; preview site pages /reference/derives, /reference, and /reference/built-in-functions.

Dependency changes

• None

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