Provide more information about using Templates (and stacks)

[rickspencer3]

We have some info in the introduction, perhaps some detailed walkthrough of exporting and importing dashboards would be helpful

[Anaisdg]

@mhall119 did we decide to exclude this information on templates because it's no longer maintained? same with stacks?

[mhall119]

@Anaisdg we excluded it from the InfluxDB Univ videos, but we didn't make a decision on including it in the book. Since it's easier to update the book, I think it could be added with that Templates support now, and then updated as it gains support for new resources