consolidate and document all `azd` environment variables

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:
- [x] Document existing
- [ ] Discuss and standardize outliers:
- [x] `AZD__TOOL_PATH`
- [ ] `AZURE_DEV_COLLECT_TELEMETRY` -> `AZD_COLLECT_TELEMETRY`
- [ ] `AZURE_DEV_USER_AGENT` -> `AZD_USER_AGENT`