docs: document C++ Doxygen group pages in library docs generator

[devin-ai-integration]

Summary

C++ library authors organize their APIs with Doxygen groups (@defgroup / @ingroup / @addtogroup) in their source, but until now the library docs generator dropped that structure: readers only saw the raw namespace and symbol hierarchy. The generator now renders each group as an index page under a groups/ folder in output.path, with the group's description and links to its members and subgroups, so a library's own conceptual organization becomes browsable in the docs with no configuration beyond the annotations already in the source. Group pages link to the existing symbol pages rather than duplicating content, and empty groups are skipped.

Implements docs for: feat(cli): render Doxygen group pages for C++ library docs (fern-api/fern#17420)

Pages changed

• fern/products/docs/pages/api-references/library-docs.mdx — new ## Doxygen groups section covering the groups/ output, nested subgroups, member links, and that empty groups are skipped and no navigation change is needed.
• fern/products/docs/pages/changelog/2026-08-26.mdx — Docs changelog entry for the feature, tagged api-reference, linking to the new section.

Note: this touches the same page as the open, unrelated PR #6700 (Doxyfile settings); this branch is off main and the diff is confined to a new section, so it should merge cleanly.

Link to Devin session: https://app.devin.ai/sessions/62fb506b8a56492286ca546dd6ed809c

[devin-ai-integration]

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[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1787763958-cpp-doxygen-groups.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/api-references/library-reference
• learn/docs/changelog/2026/8/26