Document nested request handlers

[maxime-rainville]

Following some internal discussion, we've concluded that we haven't properly documented and explained our approach no nested controllers, especially in the context of creating AJAX request endpoints.

Acceptance criteria

• A new article is added to the developer docs explaining how nested request handlers work
• A CMS developer should be able to create nested request in the CMS after reading that article
• The article is drafted with the expectation that it will be complemented with other approaches for non-nested request handlers.
• The arcticle is linked to from the routing docs

Related

• There's a bit of PHP doc comment that provides good context on this in the Request Handler class.

PRs

• DOC Nested RequestHandlers #415

[GuySartorelli]

PR merged