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/index.js

@@ -256,6 +256,18 @@ export interface PagesOptions {
   /* The URLs to the custom pages.
   :DEFAULT: {} */
   customUrls: ?PagesCustomUrlsOptions;
+  /* The custom routes.
+  :DEFAULT: [] */
+  customRoutes: ?(PagesRoute[]);
+}
+
+export interface PagesRoute {
+  /* The route path. */
+  path: string;
+  /* The route method, e.g. 'GET' or 'POST'. */
+  method: string;
+  /* The route handler that is an async function. */
+  handler: () => void;
 }
 
 export interface PagesCustomUrlsOptions {