feat: Add comprehensive cross-reference resolution system to FernRenderer

[devin-ai-integration]

feat: Add comprehensive cross-reference resolution system to FernRenderer

Summary

This PR implements a complete cross-reference resolution system for the FernRenderer to achieve parity with the MyST renderer's cross-referencing capabilities. The implementation adds automatic link generation for code references in docstrings, Anchor components for non-heading items, and infrastructure for deep linking support in code blocks.

Key Changes:

1. Reference Discovery Phase: Added _build_reference_map() that runs during initialization to build a complete mapping of all documented items to their slugs and anchors, enabling efficient cross-reference lookups.

2. Cross-Reference Resolution: Implemented _resolve_cross_reference() with multiple fallback strategies (exact match, child, sibling, module child) to resolve backtick-enclosed identifiers like `ClassName` or `function_name()` to proper Fern links.

3. Docstring Processing: Integrated _process_docstring_references() into the docstring formatting pipeline to automatically convert code references in descriptions, parameter docs, return values, and exception descriptions into clickable links.

4. Anchor Components: Added <Anchor id="..."> components to function, class, property, and data rendering methods to enable proper cross-referencing for items that don't have their own headings.

5. Deep Linking Infrastructure: Implemented _extract_code_references() to extract identifiers from code blocks and map them to documentation URLs (for Fern's deep linking feature).

Note: The deep linking method (_extract_code_references()) is implemented but not yet integrated into the rendering pipeline - this will need to be added in a follow-up if needed.

Review & Testing Checklist for Human

⚠️ Critical - This PR has not been tested with actual documentation generation. Lint checks pass, but end-to-end functionality needs verification:

• Test cross-reference resolution: Generate docs from a real Python package and verify that docstring references like `ClassName` actually become clickable links to the correct documentation pages
• Verify Anchor component syntax: Confirm that Fern supports <Anchor id="..."> syntax and that it creates properly linkable anchors (check Fern docs at https://buildwithfern.com/learn/docs/writing-content/components/anchor)
• Test link formats: Verify that same-page links use #anchor format and cross-page links use /slug#anchor format correctly
• Check for broken references: Test with a codebase that has various reference patterns (parent.Child, sibling references, etc.) to ensure resolution logic handles all cases
• Performance testing: Run on a large codebase (1000+ documented items) to verify that the initialization-time reference map building doesn't cause significant slowdowns

Test Plan Recommendation

1. Generate docs for a real Python package (e.g., a small internal package or open source project)
2. Check generated .md files for:
   • Proper <Anchor> components at the top of function/class/property definitions
   • Converted links in docstrings (search for [ `ClassName ] patterns)
   • Verify links point to correct slugs and anchors
3. Build the Fern documentation and verify links are clickable and navigate correctly
4. Test edge cases: references to undefined items should remain as plain code

Notes

• Session URL: https://app.devin.ai/sessions/d2459a345ab34986ae6dab211f6f7486
• Requested by: [email protected] (@Ryan-Amirthan)
• This PR is based on the overhaul branch (PR feat: removed all sphinx and kept it more related to Fern #6)
• The _extract_code_references() method exists but isn't called anywhere - if deep linking in code blocks is needed, this will require additional integration work
• All lint checks (ruff) pass successfully

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