Pin docs versions to get around weird error

In sphinx 4.1.2 readthedocs and local spew this error:

```
Exception occurred:
  File "/home/docs/checkouts/readthedocs.org/user_builds/moire-lattice-generator/envs/latest/lib/python3.8/site-packages/nbsphinx.py", line 2151, in depart_codearea_latex
    assert 'Verbatim' in lines[0]

```

Locally works in the sphinx version I had installed, so pinning there...

docs/requirements_docs.txt

@@ -1,6 +1,6 @@
-sphinx
+sphinx==3.5.4
 sphinx-autobuild
-sphinx_rtd_theme
+sphinx_rtd_theme==0.5.2
 ipykernel
-nbsphinx
+nbsphinx==0.8.6
 matplotlib