Generate docs for public headers in build dir

[elliotcmorris]

[plan] This file (and presumably the export.h files) don't show up in Doxygen docs. Presumably because Doxygen isn't configured to scan the build directory for our generated public headers. We should fix that, especially now we have actual functionality that's going undocumented.

Originally posted by @feltech in #1247 (comment)

We now have export.h and version.hpp in the build directory without docs being generated from them. If you squint, version.hpp does define a public interface due to the macros being useful in pipelines, so it'd be nice to doc em.

In order to do this, we'd need to enable file based documentation in our generated output, which tbh seems like a good idea.

[feltech]

I think even if we don't enable file based documentation we'll get some docs, at least for the functions. However, I agree that enabling file docs would be good.