Refactor the workflows to reduce boilerplate code

[brian-rose]

With #59 I moved the complicated code that manages the preview deployment (and associated comments in PRs) over to a reusable workflow at https://github.com/ProjectPythiaCookbooks/cookbook-actions/blob/main/.github/workflows/deploy-preview.yaml

Still to do to reduce boilerplate in each Cookbook repo and simplify maintenance:

• Similarly migrate the code for delete-preview
• Similarly migrate the code for deploy-book

[brian-rose]

And once that is all done, we will need to push these changes out to all the existing Cookbook repos.

Another example of the need for ProjectPythia/cookbook-gallery-sphinx#86

[brian-rose]

After much fussing around, this is done.

There may be some further simplifications of these workflows to tackle when we move the cookbook execution to binderbot.