[Doc] Refer to or explain the validate method on the "Pages directory" page

[SEMilfred]

On /docs/directory-structure/pages/ the validate method is only ever so briefly mentioned under the "The watchQuery property".

It would make sense to mention it as it's own section or at least as part of the "Dynamic pages" section and make sure to link to /docs/components-glossary/validate/.

The validate method can be quite essential to setting up dynamic pages properly.