Add C guides for qiskit 2.0

[javabster]

The following guides can be added for the qiskit 2.0 release:

• advanced setup -> installing just C
• advanced setup -> installing C + python (maybe can combine this guide with then first one)
• circuits and operators -> building observables with C

We should makes sure to set expectations in the intro to these pages that it is still early days for the C api and that there will be more functionality coming soon

@Cryoris will write these guides :)