Create "Middleware" article in "Internals"

Convention about failure handler in middleware should be written.

Because each failure handler is specific for each middleware, so it shouldn't implement RequestHandlerInterface. So separate failure handler interface should be created for each middleware.

Example:

interface AuthenticationFailureHandlerInterface
{
    public function handle(ServerRequestInterface $request): ResponseInterface;
}

When failure handler requires some context, it should be a second parameter in handle() metod. For example:

interface ThrowableHandlerInterface
{
    public function create(ServerRequestInterface $request, Throwable $throwable): ResponseInterface;
}

Related PRs:

• https://github.com/yiisoft/auth/pull/101

Start by reviewing the requested Internals article topic and the related yiisoft/auth pull request #101. Locate the existing Internals documentation structure, then document the middleware failure-handler convention and the AuthenticationFailureHandlerInterface and ThrowableHandlerInterface examples shown in the issue. Done means the convention is clearly recorded in the new article.