CMake: harvest definitions in modules

# Feature request

## Summary

The CMake documentation includes [pages describing bundled modules](https://cmake.org/cmake/help/latest/manual/cmake-modules.7.html). DevDocs has an index entry for each module (e.g. [CMakeDependentOption](https://devdocs.io/cmake~3.26/module/cmakedependentoption)), but not for each entity defined within (e.g. `cmake_dependent_option` macro).

Harvesting these would improve search user experience, especially when using the technique described in the [user guide](https://devdocs.io/help) under *Prefilling the search field*.

NB to implementor: a module can define more than one item and I’m not sure the markup is consistent between module doc pages.

Start by inspecting the existing DevDocs handling of bundled CMake module pages and how module entries are indexed. Compare several CMake module pages to identify the markup used for defined entities, then verify that multiple entities per module are harvested and searchable alongside the module itself.

Written by the indexing model from the issue text.