Change job init default to example.nomad.hcl and recommend in docs

[gulducat]

Historically we have recommended the file extension .nomad for HCL job specification files. This results in downstream issues like ~"Please support '.nomad' files in the HCL VS Code extension" where one solution is to append .hcl to the filename.

This formalizes that recommendation by:

• nomad job init now produces example.nomad.hcl by default:

$ nomad job init
Example job file written to example.nomad.hcl

• references to that and similar filenames are updated in website docs

This does not

• stop fmt from also formatting .nomad files
• change all our tests and such that use various *.nomad files and "files"
• update docs that reference files outside of this repo

Closes #13350

[tgross]

LGTM!

We probably should follow up with a pass over the Tutorials repo as well.