Explicitly pin sphinx in conda env (#136)

At time of merging, pip pins sphinx to 4.5 but conda doesn't, which results
in errors when building the site. Suspect that one or more of the packages on
conda-forge has different dependencies than those defined in the pyproject.toml.

Should be reverted in the future when the conda-forge feedstocks result in the
correct environment.

Author: rossbar

environment.yml

@@ -13,7 +13,7 @@ dependencies:
   - pooch
   - ffmpeg  # For gym/atari
   # For building the site
-  - sphinx
+  - sphinx<5
   - myst-nb
   - sphinx-book-theme
   - sphinx-copybutton