[Documentation] Update the "Extending CUDA-Q with a new Simulator" page

[1tnguyen]

The "Extending CUDA-Q with a new Simulator" doc page is out of date w.r.t. the current code.

For example,

• the "MySimulator.cpp" code snippet does not reflect the latest CircuitSimulatorBase interface.

• the CMakeLists.txt snippet is using an old signature of the nvqir_add_backend CMake function (missing the GPURequirements argument)

Hence, we need to review and update this page. Ideally, we should have this doc page pull code from .cpp/CMakeLists.txt files that are validated by the CI.