Fusion: update the doc to include the authentication features

[haijian-vaadin]

We have some small authentication helpers, e.g. login, logout, and InvalidSessionMiddleware, these should be documented in the Creating UI in TypeScript section.

1. Update the adding-login-form-with-spring-security to explain how to include a custom login form with vaadin-login-overlay, as done in the crm tutorial for typescript
2. Update the check-user-login page to include how to handle session expiration with InvalidSessionMiddleware.