[BUG] GenerateDocumentation (Doxygen) is not included in the VS-project

[christophKaiser]

Describe the bug
After Configure and Generate by CMake with default settings from CMakeLists.txt, the project OpenInfraPlatform.GenerateDocumentation isn't included / visible in the folder Commands of Visual Studio.

To Reproduce
Steps to reproduce the behavior:

1. In CMake, delete cache
2. Click Configure, Generate and Open Project in CMake
3. Search in the Solution Explorer of Visual Studio (folder OpenInfraPlatform/Commands, project OpenInfraPlatform.GenerateDocumentation -> it won't be there)

Expected behavior
The documentation "Doxygen for Open Infra Platform" states, that Doxygen will be enabled by default:

Open-Infra-Platform/Documentation/markdown/DoxygenHelp.md

Line 24 in 4fa2e0a

If `DOXYGEN_GENERATE_DOCUMENTATION` is checked in the CMake GUI (default = ON), CMake will try to find the doxygen and dot executables in `C/thirdparty`. Additionally, a command that builds the documentation will be added to the solution.

However, Doxygen is disabled in the top-level CMakeLists.txt:

Open-Infra-Platform/CMakeLists.txt

Line 324 in 4fa2e0a

Option(DOXYGEN_GENERATE_DOCUMENTATION "Build documentation for TUM OpenInfraPlatform." OFF)

Of course, the user can switch on this option in CMake. But I don't know, if an unexperienced user would find this option.
At least one file should be adjusted to be consistent.

Suggestion
I found the commit, which has turned off the setting in the CMakesLists.txt. Unfortunately, to me it isn't obvious why it was turned off.

On my system, I tested the CMake build process today:

1. Changed the above mentioned CMakeLists.txt to
   Option(DOXYGEN_GENERATE_DOCUMENTATION "Build documentation for TUM OpenInfraPlatform." ON)
2. Deleted cache in CMake
3. Clicked Configure, Generate and Open Project in CMake
4. Built OpenInfraPlatform.GenerateDocumentation in Visual Studio
5. The generated documentation opend automatically in a browser, like expected without noticeable side effects

If there is a background issue which is unknown to me, I would withdraw my suggestion.