Add documentation for how to build addressables content in start window

[vvuk]

The start window just says "To build this project, make sure to build the addressable content first, for information about how to do so please check the Addressables package manual."

This is super unfriendly. Should be "This project uses Addressables, and addressable content must be built first. To do this, do Foo Bar Baz. For more information about Addressables, see the Addressables package manual."