Document dotnetup user and developer environment variables separately

## Description

Create dedicated environment-variable documentation for `dotnetup`, split by audience:

1. A user-facing reference for supported `dotnetup` environment variables and supported .NET CLI variables that affect dotnetup.
2. A contributor/developer reference for diagnostics, test hooks, and implementation-only environment variables.

This follows the discussion in dotnet/sdk#55421: https://github.com/dotnet/sdk/pull/55421#discussion_r3705804171

The telemetry documentation currently contains a `Related Environment Variables` section added or updated as part of #55421. That information should be reorganized rather than lost:

- `DOTNET_CLI_TELEMETRY_STORAGE_PATH`
- `DOTNET_CLI_TELEMETRY_SHUTDOWN_TIMEOUT_MS`
- `DOTNETUP_TELEMETRY_FORCE_LOCAL_DELIVERY`

The audit should cover all environment variables consumed by dotnetup, including direct string lookups and constants. Current examples include:

- User/runtime behavior: `DOTNET_CLI_TELEMETRY_OPTOUT`, `DOTNET_NOLOGO`, `DOTNET_CLI_UI_LANGUAGE`, `VSLANG`, `DOTNET_DOTNETUP_DATA_DIR`, `DOTNET_CLI_TELEMETRY_SHUTDOWN_TIMEOUT_MS`, and `DOTNETUP_TELEMETRY_FORCE_LOCAL_DELIVERY`.
- Developer/diagnostic behavior: `DOTNETUP_TELEMETRY_DEBUG`, `DOTNETUP_CLI_GET_PERF_TRACE`, `DOTNETUP_DEV_BUILD`, telemetry exporter overrides, and disk logging.
- Test-only hooks: `DOTNET_CLI_TELEMETRY_E2E_CONNECTION_STRING`, `DOTNET_TESTHOOK_DOTNETUP_TELEMETRY_SHUTDOWN_BUDGET_PATH`, `DOTNET_TESTHOOK_MANIFEST_PATH`, and `DOTNET_TESTHOOK_DEFAULT_DOTNET_PATH`.

Variables owned and already documented by the .NET CLI should link to the canonical CLI documentation instead of duplicating definitions that may drift. The documentation should explicitly decide where `DOTNET_CLI_TELEMETRY_STORAGE_PATH` belongs if it is not already covered canonically.

The shutdown-timeout and force-local-delivery controls should remain discoverable to users. They allow CI users to choose the latency/reliability tradeoff rather than disabling telemetry entirely, and force-local delivery may later be useful for GitHub Actions or Azure DevOps task integration.

## Acceptance criteria

- Add a consolidated user-facing `dotnetup` environment-variable reference in the canonical dotnetup documentation area.
- Add a separate contributor/developer environment-variable reference for diagnostics and test hooks.
- Audit dotnetup source and tests for all consumed environment variables and classify each as user-facing, shared .NET CLI, developer/diagnostic, or test-only.
- Link shared .NET CLI variables to their canonical documentation instead of duplicating their contracts.
- Update `src/Installer/dotnetup.Library/docs/dotnetup-telemetry.md` so its environment-variable content links to the new references and remains focused on telemetry behavior and privacy.
- Preserve documentation for the CI shutdown timeout and force-local-delivery controls in the user-facing reference.
- Ensure documentation distinguishes supported public controls from implementation details and test hooks.