Explain relationship between template URLs and stacks.

Templates and stacks docs (https://docs.influxdata.com/influxdb/cloud/influxdb-templates/stacks/init/) provide several examples, but the relationship between stacks and template s isn't immediately clear. A scenario that walks through a sequence of actions might help.

- What happens when I `init` a stack?
- When would I `apply` a template vs `init` a stack?
- What are template URLs used for when I create (`init`) a stack?
- Add API examples for stacks and templates.

##### Relevant URLs
- https://docs.influxdata.com/influxdb/cloud/influxdb-templates/stacks/init/

Start with the relevant stacks init page at https://docs.influxdata.com/influxdb/cloud/influxdb-templates/stacks/init/ and review its existing template and stack examples. Trace the documented init and apply actions, then add a scenario explaining their relationship and API examples covering the requested questions; the page should clearly show when template URLs are used and how stacks are created.

Written by the indexing model from the issue text.