Turn off parallel docs build and pin dependencies (#1097)

### Summary

The docs build is suddenly breaking with 

```
  File "/home/runner/work/qiskit-experiments/qiskit-experiments/.tox/docs-parallel/lib/python3.8/site-packages/sphinx/util/parallel.py", line 126, in _join_one
    raise SphinxParallelError(*result)
sphinx.errors.SphinxParallelError: sphinx.errors.ExtensionError: Unable to find kernel (exception: No such kernel named python3)
```

Turning off the parallel flag didn't solve the problem, but pinning
`ipykernel` and `jupyter-client`, both of which put out new releases
yesterday, solved the problem. I'm keeping the parallel flag off since
it tends to cause issues until this compatibility issue is resolved.

.github/workflows/main.yml

@@ -97,7 +97,7 @@ jobs:
           python -m pip install -U tox
           sudo apt-get install -y pandoc graphviz
       - name: Build Docs
-        run: tox -edocs-parallel -- -W
+        run: tox -edocs -- -W
       - name: Compress Artifacts
         run: |
           mkdir artifacts

requirements-dev.txt

@@ -21,3 +21,6 @@ pylatexenc
 importlib-metadata==4.13.0;python_version<'3.8'
 scikit-learn
 sphinx-copybutton
+# Pin versions below because of build errors
+ipykernel<=6.21.3
+jupyter-client<=8.0.3