Document "new-tab" keyword in menu configuration

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:

```yaml
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](https://fshschool.org/docs/sushi/configuration/#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](https://fshschool.org/docs/sushi/configuration/) page as well.