Document C/C++ macro pages, include-undocumented-macros, and plain-C index pages in library docs

[fern-api]

Summary

The library docs generator now renders C/C++ preprocessor macros (#define) as reference pages, so macro-heavy C and C++ SDKs get complete API reference instead of silently dropping macros. By default only macros with a Doxygen doc comment get pages, and a new include-undocumented-macros option emits every macro. Plain C libraries (symbols at global scope, no namespace) now get library and category index pages, and \ref/@see references inside Doxygen groups render as working links.

Implements docs for: feat(cli): C/C++ library docs — macro pages, plain-C index pages, group-scoped cross-links (fern-api/fern#17789), feat(fdr): C/C++ library docs — macro extraction, function-pointer typedefs, post-deploy test (fern-api/fern-platform#14951)

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

• Define your libraries step: both docs.yml examples show include-undocumented-macros: true # optional, C++ only under config.
• Generate the library docs step: a sentence noting that \ref/@see references inside groups link to symbol pages, plus a new paragraph on macro pages (Macros category, group pages, documented-only default, include guards skipped) and plain-C index pages.
• Configuration reference: new config.include-undocumented-macros ParamField (boolean, default false, C++ only). The YAML key is kebab-case, matching the LibraryConfig schema in fern#17789.

The intro paragraph is unchanged from main.

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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• learn/docs/api-references/library-reference