[Docs] Document the photonics backend

[anthony-santana]

Required prerequisites

• Search the issue tracker to check if your feature has already been mentioned or rejected in other issues.

Describe the feature

Add a page under the backends index for the photonics backend + add it to the list in backends.rst.

[khalatepradnya]

Completed with #2502.

Current location: https://github.com/NVIDIA/cuda-quantum/blob/f662540b96e2546b014b1aa729b093565593233e/docs/sphinx/using/backends/sims/photonics.rst

See: https://nvidia.github.io/cuda-quantum/latest/using/backends/sims/photonics.html#orca-photonics