Update Nomad variables page to add tutorial links for variables

### Proposal
Currently, [Nomad variables concept page](https://developer.hashicorp.com/nomad/docs/concepts/variables) is not first-time-user-friendly.

We should at least add tutorial links and [variables command](https://developer.hashicorp.com/nomad/docs/commands/var) in this page.
We can also add more sections to talk about how variables work, instead of just the ACL part.

Open the Nomad variables concept page linked in the issue and review the linked variables command documentation and available tutorials. Done means the concept page includes tutorial links and a link to the variables command; any additional explanation should stay within the page’s variables scope.

Written by the indexing model from the issue text.