Add a cross field validation page in the docs

[ealush]

Based on this issue, #475, I realized there's a need to describe the different ways to have a cross field validation. The ways are briefly described in the issue, but they need to be turned into a documentation page.

The main scenarios:
Either/Or validations: At least one field needs to be validated
And validations: A field can only be validated if another field has been validated

Please submit a PR with the docs if you want to take it.