Add more detailed documentation/demo about default behavior after login

[DiegoCardoso]

When user submits the login, the submit button is disabled to prevent another attempt before the login is processed.

Add:

• Documentation describing the behavior
• Demo with details with what the developer should do in order to re-enable login

[web-padawan]

Moving this to the docs repository. This needs to be handled when creating new demos for the component.

[jouni]

There is now textual documentation about this: https://vaadin.com/docs/latest/components/login/#login-event

Should probably still implement a proper code example.

[jouni]

I think the first example should already tell about this behavior, and the example should show how to re-enable the button once the login attempt has been handled (either valid or invalid credentials).