post-processing: consolidate scripts, reduce use of global state

[jameslamb]

Contributes to rapidsai/build-planning#197 and https://github.com/rapidsai/ops/issues/4044

In #657 (and previously in #654), I'm working on adding an "Inactive Projects" section to https://docs.rapids.ai/api/, and currently struggling to make that work with the post-processing that populates drop-downs like this:

ref: https://docs.rapids.ai/api/cudf/stable/user_guide/

To try to make this more manageable to both change and review, this PR proposes some refactoring:

• copying the contents of ci/customization/util.py into the only place they're used (to eliminate the indirection of an import)
• switching most functions from relying on global state to instead passing configuration through keyword arguments

This should ONLY be a moving-code-around PR, it isn't intended to change anything about how the docs look or work.

Notes for Reviewers

I'm sorry, I know this has not been the most fun series of PRs to review. Thanks for your patience.

How I tested this

Using the steps added to the docs in #659

./ci/customization/customize_docs_in_folder.sh "_site/api/cudf"

Beyond that, we'll have to merge this to find out if it worked.

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[ajschmidt8]

you deserve a 🍪 for wrangling this mess.

[jameslamb]

Ha no prob, thanks for reviewing! I'll merge this so we can get some feedback from a deployment.