Docs: Refactor "Environment variables" into 3 articles (Diátaxis)

[benjaoming]

This breaks up our current Environment Variables guide:

• Avoids mix of "pre-defined" environment reference with "user-defined" scope
• Replaces "user-defined" with "custom"
• Adds more guidance in howto + a screenshot
• Discusses alternatives to non-secret environment variables

Refs: #9746

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📚 Documentation previews 📚

• User's documentation (docs): https://docs--9966.org.readthedocs.build/en/9966/

• Developer's documentation (dev): https://dev--9966.org.readthedocs.build/en/9966/

[benjaoming]

@ericholscher I dare say that this is ready for review 👍

[ericholscher]

I'm a little confused on the differences in these 2 pages. I think a better title on Configuring environment variables would help. It's not about configuring them, is it?

[benjaoming]

I'm a little confused on the differences in these 2 pages. I think a better title on Configuring environment variables would help. It's not about configuring them, is it?

It was a terrible title!

There are currently 3 pages in play, all of which are largely finished methinks..

1. A how-to about defining environment variables for a project
2. A reference to built-in environment variables that are available in the build process
3. An explanation about accessing environment variables in the build process

The "3rd page" was called "Configuring environment variables" and the title wasn't good. I've changed it to "Understanding environment variables" and added a better introduction and some more content about sphinx-multiproject, since that AFAICT is a very solid reason to mention here.

[ericholscher]

This is a 💯 refactor. Thanks for explaining it. Definitely quite close, just a few nitpicks.