Fix qcvv/xeb_theory.ipynb as part of docs cleanup for Cirq 1.0 (#5202)
Fix qcvv/xeb_theory.ipynb as part of docs cleanup for Cirq 1.0 (#5202)

* Moved import to top
* Fixed broken link to quantum ai website
* Minor improvements in text formatting
quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq