Improve instructions for using add-ons using Quarkus

[samie]

Using Vaadin Add-ons with Quarkus need special configuration for the development build.
See: https://vaadin.com/docs/latest/integrations/quarkus#quarkus.vaadin.addons

It is not obvious and the resulting error message does not indicate what might be wrong.
More instructions should be added to Directory avoid the situation.

Few ideas moving forward:

• Add Quarkus information/link to Directory install dialog
• Improve add-on template to include Jandex Index generation by default.
• Generate Jandex Index for all add-ons automatically.