Let's consolidate and document all existing azd environment variables for our community and users.

This is primarily an "under-the-covers" documentation, preferably we should have a pristine copy of this in the codebase repository. We can also expand our footprint to our learn docs.

The final stretch goal: we should be able to surface in-terminal help, an idea is presented in #4319

Tasks:

• Document existing
• Discuss and standardize outliers:
  • AZD_<tool>_TOOL_PATH
  • AZURE_DEV_COLLECT_TELEMETRY -> AZD_COLLECT_TELEMETRY
  • AZURE_DEV_USER_AGENT -> AZD_USER_AGENT