Add tutorial on circuit optimization, gate alignment, & spin echoes (#…

…4373)

See tutorial intro for full description. Briefly, this tutorial explains how to use circuit optimizers and insert spin echoes to improve circuit fidelity as discussed in the [best practices guide](https://quantumai.google/cirq/google/best_practices#improving_circuit_fidelity), and shows a benchmark.

cc @tanujkhattar

docs/_book.yaml

@@ -193,6 +193,8 @@ upper_tabs:
         path: /cirq/tutorials/google/visualizing_calibration_metrics
       - title: "Qubit picking with Loschmidt echoes"
         path: /cirq/tutorials/google/echoes
+      - title: "Circuit optimization, gate alignment, & spin echoes"
+        path: /cirq/tutorials/google/spin_echoes
       - title: "Calibration"
         style: accordion
         section:

docs/google/best_practices.md

@@ -202,6 +202,8 @@ after gates in previous moments.  Many of these tips focus on having a
 good moment structure that avoids problematic missteps that can cause
 unwanted noise and error.
 
+Note: See the [Circuit optimization, gate alignment, and spin echoes tutorial](../tutorials/google/spin_echoes.ipynb) for an example of the best practices discussed in this section.
+
 ### Short gate depth
 
 In the current NISQ (noisy intermediate scale quantum) era, gates and devices still