Add custom routes to pages router (#7231)

* added custom routes

* fixed docs typos

* added page.customRoutes config validation

* added 404 response if missing custom route response

* added docs

* minor README formatting

* added CHANGELOG entry

* fixed bug in definitions builder that did not recognize array of custom type

* added missing route handler definition

* fixed custom routes definition

src/Options/docs.js

@@ -82,6 +82,7 @@
 
 /**
  * @interface PagesOptions
+ * @property {PagesRoute[]} customRoutes The custom routes.
  * @property {PagesCustomUrlsOptions} customUrls The URLs to the custom pages.
  * @property {Boolean} enableLocalization Is true if pages should be localized; this has no effect on custom page redirects.
  * @property {Boolean} enableRouter Is true if the pages router should be enabled; this is required for any of the pages options to take effect. Caution, this is an experimental feature that may not be appropriate for production.
@@ -93,6 +94,13 @@
  * @property {Object} placeholders The placeholder keys and values which will be filled in pages; this can be a simple object or a callback function.
  */
 
+/**
+ * @interface PagesRoute
+ * @property {Function} handler The route handler that is an async function.
+ * @property {String} method The route method, e.g. 'GET' or 'POST'.
+ * @property {String} path The route path.
+ */
+
 /**
  * @interface PagesCustomUrlsOptions
  * @property {String} emailVerificationLinkExpired The URL to the custom page for email verification -> link expired.