Fix docs/circuits.ipynb as part of docs cleanup for Cirq 1.0 (#5177)
Fix docs/circuits.ipynb as part of docs cleanup for Cirq 1.0 (#5177)

- Remove output
- Move import to top
- Minor changes to content (add `cirq.` prefix to many quoted words)
- Remove broken link to devices.ipynb
quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq