This diff decouple the docs requirements from the dev_install.
So, for example, dagster-airflow api docs can be built properly (it requires both dagster-airflow and airlfow but we can't install airflow in dev_install as discussed in D6206)
We will use pip install -r docs-requirements.txt instead of make dev_install to install docs dependency
I updated the "Publish the docs" section too to reflect the change.
In the long term, the plan with the new site is to use a docker image to release the docs so it won't be affected by individual local virtualenv.