Document Cython .pyx module support in the Python library docs generator

[fern-api]

Summary

The Python library docs generator now documents Cython .pyx modules automatically. Before this change, Cython-heavy packages had incomplete reference docs unless the authors maintained .pyi stubs. Any .pyx file without a .py or .pyi file of the same name now gets pages for its Python-visible API, with no configuration. A module that fails to parse is skipped with a CLI warning, and the rest of the library is still documented.

Implements docs for: feat(fdr): native Cython .pyx support in Python library docs parser (fern-api/fern-platform#14949)

Changes to fern/products/docs/pages/api-references/library-docs.mdx

• New ## Cython modules section placed after Customize generated docs and before Configuration reference. It no longer sits between the intro and Configuration. It covers:
  • When .pyx files are documented (no sibling .py/.pyi).
  • What's included: class/cdef class, def/cpdef with full signatures, cdef public/cdef readonly attributes and properties, docstrings, __all__, constants, and imports.
  • What's excluded: cdef functions, private cdef attributes, ctypedef, cimport, and include. C types are mapped to Python types only where the mapping is unambiguous.
  • Parse failures: the module is skipped and fern docs md generate prints a non-fatal warning. Verified against orchestrate.ts in fern-api/fern (CLI 5.144.2).
• Removed the implementation details (stub synthesis and Griffe) and the exact warning string.

Link to Devin session: https://app.devin.ai/sessions/f58dda5bf6eb40d8bad5ee9284244ce9
Open in Devin Desktop: https://app.devin.ai/desktop/session/f58dda5bf6eb40d8bad5ee9284244ce9?variant=devin

[github-actions]

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