Add module level docstrings

[david-pl]

Right now, we have no module level docstrings in the respective __init__.py files. This leads to the API reference, which is autogenerated, to show an empty page on submodules. For example, here https://bloqade.quera.com/latest/reference/bloqade-circuit/src/bloqade/analysis/

We should add these throughout simply for the benefit of having a nicer API reference. Each description should inlcude

• A brief summary of what the module focuses on
• A short list of the functionality there is. This can mention classes, but listing them doesn't make too much sense since the list can become outdated and they are already listed in the navigation.
• If applicable, a short example would be nice.