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.