…eck()` can now register a callback for specific item or attribute, which should improve performance when using custom callback checks. Callback checks should be added only for specific item or attribute if possible.

Fix (engine): `Schema#checkChild()` will now correctly check custom callback checks for each item in the context.

Internal: Used specific callback checks in some of the features.

Docs (engine): Improved schema deep dive guide and added missing section related to attributes custom callback checks.

…d API docs.