Add environment variables docs page

[stdavis]

Closes #8856

Summary

Provide a central list of all environment variables set by Jest. I searched through the jest code and think that I found all of the variables that it sets. Please let me know if I missed any.

Test plan

I've previewed the changes locally on the website and they look good to me.

[SimenB]

Wonderful, thanks! I'll play with this tomorrow so it's available across versions but this is great! 👍

[stdavis]

I'd be happy to push it to other versions if you'd like. Should it be pushed to all of them?

[jeysal]

Yep, both of those variables have been around for ages, so all of them. Don't forget to update both versioned_docs and versioned_sidebars since this is one of the rare cases where the latter also changed

[stdavis]

Let me know if I missed something. I'm so happy to help contribute (in a tiny way) to a super-awesome tool that I use every day.

[SimenB]

Docusaurus picks up the version of the docs from a previous release, so we just need a "master" version in docs/ and in the oldest "versioned version" of the docs in addition to the sidebar

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.
Please note this issue tracker is not a help forum. We recommend using StackOverflow or our discord channel for questions.