Generated README conda update instructions lead to conflicts and slow updates

[ianmkenney]

The generated docs instruct you to update a fresh environment ($CONDAENV) with the following:

conda env update --name $CONDAENV --file devtools/conda-envs/test_env.yaml
conda env update --name $CONDAENV --file docs/requirements.yaml

Currently, separating the two steps leads to package conflicts (on Linux). If using both anyway, you might as well update in the same command.

conda env update --name $CONDAENV --file devtools/conda-envs/test_env.yaml --file docs/requirements.yaml

This nearly halves the update time as conflicts are resolved before they even occur. Even so, this process takes around 20 minutes in a fresh environment. As I don't normally use conda, I'm not sure if that's out of the norm.