Add delimiter escape info in Documentation

### Proposal
Add delimiter escape info in Documentation.

In template I have an env like this:
`URL = http://1.1.1.1:1111/clients/{{ID}}`
where {{ID}} is later being replaced in Go templates, but during deploying I get the following error
`Template failed: (dynamic): parse: template: :12: function "ID" not defined`
I've searched info in https://www.nomadproject.io/docs/job-specification/hcl2/expressions and https://www.nomadproject.io/docs/job-specification/template but couldn't find.

After many trials next option worked
`URL = http://1.1.1.1:1111/clients/{{"{{ID}}"}}`

It looks like a common solution for nomad seniors, but not for novices like me. My proposal is to add this case in documentation. Thank you

Start with the Nomad documentation pages for job-specification/hcl2/expressions and job-specification/template, which the issue identifies as the relevant entry points. Add the documented delimiter-escaping example for a URL containing {{ID}}, explain why the plain form is parsed as a template function, and verify that the guidance matches the reported working form.

Written by the indexing model from the issue text.