[DEVOPS] Add FindDoxysphinx.cmake file

### DevOps Request

Error handling is easier when Doxysphinx in the documentation process is also searched by FindPackage in CMake.

### Description of the proposed solution

Create a FindDoxysphinx.cmake similar to the FindSphinx.cmake file. Use find_package(Doxysphinx) in doc/CMakeLists.txt and the created variable for the doxysphinx executable.

### Additional context

Nothing.
