[Docs] Add environment variable reference page

[adeljo-amd]

Motivation

We've introduced a standard for environment variables to adhere to a format and to be placed in a page of its own - so it can be linked here https://rocm.docs.amd.com/en/latest/reference/env-variables.html#environment-variables-in-rocm-libraries

Technical Details

Only docs changes - move environment variables to amdsmi-env-var.md

Test Plan

Test Result

Submission Checklist

• Look over the contributing guidelines at https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.