Use Cookbook build and preview infrastructure

[brian-rose]

Here's a first attempt to harmonize the GitHub actions workflows for this repo with what we're doing in our Cookbook and Foundations repos.

[brian-rose]

Yes, environment caching works here too, and substantially speeds up the build time.

[brian-rose]

Specific changes introduced in this PR:

• Outsourcing a lot of infrastructure code to the Cookbook Actions repo (now used by Foundations too)
• Caching of the conda/mamba environment (as we already do for other repos) to speed up build times
• Previewing on GitHub pages, removing dependence on third-party Netlify deployment for previews
• Introducing a nightly build action for continuous testing (we haven't had this for this repo before)
• Removing obsolete documentation about the Netlify deployment

This should ease our ongoing maintenance burden by aligning this repo's infrastructure with Cookbooks and Foundations.

[brian-rose]

The previewing won't work until / unless this gets merged due to the fact that those actions need to run from the host branch. But the build is producing correct-looking artifacts (e.g. here) and the previews should work exactly as they do in our Cookbook repos.

[brian-rose]

Still to do, but in a different PR:

Add link-checking to our tests.

[brian-rose]

Also if everything in this PR is working properly, then we can/should make similar changes to the workflows in the Cookbook Gallery repo, which is also built with sphinx.