loosen the docs requirements

In docs/requirements.txt, we have many packages pinned to only a specific version.

Goal: figure out as many of these packages where we can replace the == with >= to ensure we're running the docs build with the latest versions in our CI.

Why? Because the docs build auto-runs all of our tutorial notebooks and we'd like to automatically make sure the tutorials run with the latest versions of the dependency packages (and be aware when a dependency has released a new version that breaks things).

If working on this, you can feel free to only focus on a specific dependency and test the docs build if you replace == with >= for that one dependency.