[docs] Missing cudaq::{observe, sample, get_state} entries in C++ API docs

[lmondada]

Hi CUDA-Q team!

Thanks for this amazing project and the great work! It's been very fun and easy to get into it.

Looking at the C++ API docs, I've noticed that the functions cudaq::observe, cudaq::sample and cudaq::get_state (there might be more) do not have an entry. Are they listed on some other page I might have missed?

This is a minor issue, as the Python equivalents of these functions are well documented. If I have a moment tonight I might try to fix this myself.