_content/doc: add information about telemetry env vars

Add documentation to doc/telemetry.md about the GOTELEMETRY and
GOTELEMETRYDIR environment variables.

Fixes golang/go#68928

Change-Id: I801be3943df515bef3b096ca2e21f43026d9aa02
Reviewed-on: https://go-review.googlesource.com/c/website/+/616916
Reviewed-by: Michael Matloob <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: findleyr

_content/doc/telemetry.md

@@ -130,6 +130,13 @@ With Go 1.23 or later, the following commands interact with the telemetry mode:
 - `go telemetry off`: set the mode to `off`.
 - `go telemetry local`: set the mode to `local`.
 
+Information about telemetry configuration is also available via read-only Go
+environment variables:
+
+- `go env GOTELEMETRY` reports the telemetry mode.
+- `go env GOTELEMETRYDIR` reports the directory holding telemetry configuration
+  and data.
+
 The [`gotelemetry`](/pkg/golang.org/x/telemetry/cmd/gotelemetry) command can
 also be used to configure the telemetry mode, as well as to inspect local
 telemetry data. Use this command to install it: