Add link to cookbook section for SecurityHeaders

So it shows up in the class API docs under https://api.cakephp.org/3.7/class-Cake.Http.Middleware.SecurityHeadersMiddleware.html
Just like on https://api.cakephp.org/3.7/class-Cake.Http.Cookie.Cookie.html

src/Http/Middleware/SecurityHeadersMiddleware.php

@@ -20,6 +20,8 @@ use Psr\Http\Message\ServerRequestInterface;
 
 /**
  * Handles common security headers in a convenient way
+ *
+ * @link https://book.cakephp.org/3.0/en/controllers/middleware.html#security-header-middleware
  */
 class SecurityHeadersMiddleware
 {