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

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

@@ -162,6 +162,12 @@ navigation:
 ```
 </Accordion>
 
+## Doxygen groups
+
+For C++ libraries, the groups your source defines with `@defgroup`, `@ingroup`, or `@addtogroup` are rendered as index pages in a `groups/` folder inside `output.path`, alongside the generated namespace folders. Each group page holds the group's Doxygen description, links to its members organized by kind, and links to its subgroups; nested subgroups become nested folders. Member documentation stays on the symbol pages, so group pages don't duplicate content.
+
+Groups whose entire subtree has no members are skipped. The `folder:` navigation entry pointed at `output.path` picks up the group pages, so no navigation change is required.
+
 ## Customize generated docs (optional)
 
 You can reorganize the output directory to restructure the sidebar navigation. Move, rename, or nest files and subfolders, and Fern picks up the new layout on the next `fern docs dev` or publish. 

fern/products/docs/pages/changelog/2026-08-26.mdx

@@ -0,0 +1,7 @@
+## Doxygen group pages for C++ library docs
+
+<ChangelogTags>api-reference</ChangelogTags>
+
+The library docs generator now renders the Doxygen groups your C++ source defines with `@defgroup`, `@ingroup`, or `@addtogroup` as index pages in a `groups/` folder, so your library's own grouping shows up in the docs alongside its namespaces. Group pages carry the group description and link to each member's page, and nested subgroups become nested folders. No extra configuration is required beyond the annotations already in your source.
+
+<Button intent="none" outlined rightIcon="arrow-right" href="/learn/docs/api-references/library-reference#doxygen-groups">Read the docs</Button>