docs(library-docs): document supported Doxyfile settings and Doxygen group pages

[devin-ai-integration]

Summary

Two C++ library-docs behavior changes, consolidated into one PR (absorbs #6757):

Doxyfile allowlist (fern-api/fern-platform#14062): Fern no longer hands a customer's Doxyfile to doxygen as-is. It reads only ALIASES, PREDEFINED, FILE_PATTERNS, OPTIMIZE_OUTPUT_FOR_C into its own doxygen configuration and ignores everything else. Settings that used to work (notably INPUT and @INCLUDE) are silently dropped, and invalid values for the four supported settings fail the publish. The page's existing claim that Doxyfile input directives widen the source root was wrong and is corrected.

Doxygen groups (fern-api/fern#17420): the generator now renders @defgroup / @ingroup / @addtogroup as index pages under groups/ in output.path.

Pages changed

• fern/products/docs/pages/api-references/library-docs.mdx
  • config.doxyfile in the configuration reference lists the four honored settings, states that everything else (including INPUT, INPUT_FILTER, GENERATE_XML, @INCLUDE, @INCLUDE_PATH) is ignored, and describes what makes a value invalid (spaces/shell metacharacters in FILE_PATTERNS, non-YES/NO booleans, malformed alias definitions, trailing backslashes, $(VAR) expansion) plus the 262,144-character limit.
  • Corrected the C++ source-root note (rebased over docs: --local supports git library inputs #6736's git-input wording): the parser uses the source root (input.path directory or repo root for git); Doxyfile input directives have no effect.
  • Doxygen group output is described in the "Generate" step alongside the existing sentence on what the generated docs contain.

No changelog entries (removed per review).

Link to Devin session: https://app.devin.ai/sessions/db14f54a90df4591bdb133f8cd657148
Open in Devin Desktop: https://app.devin.ai/desktop/session/db14f54a90df4591bdb133f8cd657148?variant=devin

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[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1787245687-doxyfile-allowlist.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

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