Document `Cypress.ensure`

[BlueWinds]

Subject

API - Cypress.ensure

Description

In cypress-io/cypress#24697, we created a new public API, ensure, on the global Cypress object. We mentioned this new API in #4835, but did not fully document it. We should add a page for Cypress.ensure same as for all our other exposed APIs.

[emilgoldsmith]

Also, the current Custom Queries#Validation section is purely referencing deleted methods that don't work and should reference Cypress.ensure, I only figured out the problem by diving into Cypress source code and finding what was used internally

[DamienCassou]

This documentation is important.

[BlueWinds]

Sorry for leaving this so long without any motion. The docs are written up in the linked PR.