Introduce `QuantumRegister` and `ClassicalRegister`

[kaelynj]

URL, if applicable

https://docs.quantum.ibm.com/guides/construct-circuits)

Describe the new content you are requesting.

Looking over the docs for constructing circuits, I don't see any discussion of the QuantumRegister or ClassicalRegister objects, save for the section on arithmetic circuits in the circuit library page. Exposing this feature to users outside of the API docs is needed imo. This could easily be included by incorporating their usage in the "Construct circuits" page.

If this new content request is accepted, do you want to write the content?

I will write a draft of the proposed content

[shraddha-aangiras]

Hi, I have introduced them in this PR!