Automatic main menu documentation

[platosha]

We should have an article explaining how the automatic main menu works. The aspects to mention:

• How the menu entries are collected:
  • In Flow, using the @Menu annotation,
  • In Hilla, using the file router views metadata.
• What kind of sorting and filtering is applied.
• How the user authentication affects the menu with various view access control rules.
• How to get the data for menu items and render the menu:
  • createMenuItems() function in Hilla
  • Equivalent for Flow

Update creating the main menu guide: https://vaadin.com/docs/next/flow/application/main-view#flow.application.main-view.menu

[taefi]

The Creating Menu From Routes heading is partially addressing this issue. Probably, some points regarding the filtering/ordering of the menus can be covered under the same section (not to detailed though), and a separate article at the Platform level docs can cover the topic in more details also addressing the hybrid applications.