Doxygen documentation

The intent is to generate documentation only for functions that are designed to be part of the public API and therefore documented with Doxygen comments. No documentation will be generated for code without such comments. Note that Doxygen only generates documentation for free functions if they are in a file with a @file comment.

Update the README to point at the documentation that will be generated and hosted by GitLab Pages.

The main page generated by Doxygen shall become the index page for the IntrinsicCV GitLab Pages page. Therefore include a link from that main page to the coverage page.

Doxygen doesn't handle VA_OPT well so split INTRINSICCV_BINARY_OP into two more specialised macros.

Add @copydoc comments to generate documentation for similar functions with minimal copy-paste.

GitLab supports customising the directory from which pages are uploaded so use a build subdirectory instead of public, to limit how messy the source folder gets.

Add Doxygen to Dockerfile. Doxygen may use GraphViz to create graphs so install that as well.

While updating the Dockerfile it was found that it wouldn't build because LLVM is now at version 19 so fix the LLVM version.