This should get test-documentation and test-help-documentation passing
again after we moved everything around in the last commit.
There are a lot more absolute links (generally in code comments) to
update, but they are lower-priority and can be done in a follow-up
pass.
Thanks to Rishi Gupta, James Rowan, and many others for their work on
this.
This replaces most of the content in prod-customize.md; we'll
deduplicate soon.