use --no-deps when installing dagster reqs during docs build, use automock when building sphinx docs

[gibsondan]

use --no-deps when installing dagster reqs during docs build, use automock when building sphinx docs

Summary:
This ended up being much more of a journey than I was hoping, and I'm not sure the cost is worth the benefit.

For reasons that are not clear to me, this new setup also surfaced more warnings that needed to be cleaned up.

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[gibsondan]

Current dependencies on/for this PR:

• master
  • PR Treat sphinx warnings as errors in docs build #5138
    • PR use --no-deps when installing dagster reqs during docs build, use automock when building sphinx docs #5156 (this PR)

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[gibsondan]

Filed an issue against sphinx here to try to understand why on earth all these new warnings popped up: sphinx-doc/sphinx#9733

[gibsondan]

and an issue against sphinx-click to try to understand why mocking doesn't seem to work with click entry poitns: click-contrib/sphinx-click#86 (although I increasingly think we won't bother with trying to get mocking working)

[yuhan]

Q management: let's pick this back up later when we/they figure out the mysterious warnings