Document "new-tab" keyword in menu configuration

[jafeltra]

In the menu configuration in a sushi-config.yaml file, authors can use the keyword "new-tab" to configure a menu page to open in a new tab.

For example, the following configuration is supported by SUSHI and will configure the generated menu to open the "Example" menu item in a new tab:

menu:
  IG Home: index.html
  Example: new-tab https://example.com/

The keyword "new-tab" is not documented on FSH School right now. It would be helpful to document the "new-tab" keyword in the table on FSH and IG Processing: Full Configuration. It may even be helpful to include the "new-tab" keyword in the exhaustive-config.yaml file included on the Configuration page as well.