Documentation: Describe how to use CMakePresets with custom CMake arguments

[ADKaster]

When building for multiple branches at once, developers might want to have several parallel build directories.

This can be accomplished by invoking cmake directly. The CMakePresets.json provides a template for each build type, but developers can pass custom cmake flags to each invocation as well.

cmake --preset Debug -DCMAKE_CXX_COMPILER=clang++-18 -DCMAKE_C_COMPILER=clang-18 -B Build/some-feature -DENABLE_QT=ON
cmake --build Build/some-feature

For example.

Explain this in the build documentation and provide examples for how --preset interacts with custom build flags.