Build BornAgain with Doxygen support

Configure with

cmake .. -GNinja -DCONFIGURE_DOXY=ON

or

cmake .. -GNinja -DDEV=ON

Then build the BornAgain API docs with

ninja doxy

Equivalently, use CMake’s build command:

cmake --build . --target doxy

The generated files are written to the html subdirectory of the build directory.

To inspect these docs, point your browser to

file://<absolute build directory path>/html/index.html