Improve Spring documentation based on DX feedback

[pleku]

• https://github.com/vaadin/flow/blob/master/flow-documentation/spring/tutorial-spring-basic.asciidoc#spring-boot
  • Add a link to Spring Boot official docs
  • Clarify that the @SpringBootApplication handles the magic of making your app bootful
• https://github.com/vaadin/flow/blob/master/flow-documentation/spring/tutorial-spring-basic.asciidoc
  • Split the Spring Boot and Spring Web Application to separate pages (move web app to its own page) and add clarity on the beginning that this it for using X, see link-to-Y for foobar.
• https://github.com/vaadin/flow/blob/master/flow-documentation/spring/tutorial-spring-scopes.asciidoc
  • Add a link to the Vaadin Session documentation which explains what is Vaadin Session Create tutorial/documentation about VaadinSession, UI #3177
  • Add a link to the UI lifecycle documentation which explains the UI lifecycle
• https://github.com/vaadin/flow/blob/master/flow-documentation/spring/tutorial-spring-scopes.asciidoc
  • Remove the Bean interface, and instead just have two Service classes with both Session and UI scoped (no need to have the interface definition there)
  • Explain that the services are now for the ui/session
• Add a list of things to the README for the tutorial that list the things the tutorial has
  • e.g. adding a service initialization listener

[denis-anisimov]

#3336 is created for the scopes improvements to be able to unblock this ticket and make all other changes.

[denis-anisimov]

Issue moved to vaadin/spring #276 via ZenHub