Document QUnit.equiv()

[Krinkle]

Has existed since 1.0.0. I'd consider it part of the "Extension interface" category.

Site source is in docs/. Get started locally: https://github.com/qunitjs/qunit/tree/main/docs#readme

See /test/main/equiv.js for examples of how it is works and how it is used.
See also https://qunitjs.com/api/assert/deepEqual/, which is powered by this utility.