Add VIAM_LOGFILE and VIAM_NO_WINDOWS_EVENT_LOGGER env vars to viam-server reference

RDK #5971 added two new environment variables for viam-server logging
configuration. Document them in a new Environment variables section on the
viam-server reference page.

https://claude.ai/code/session_01YLbpqo3cWMJErcD1KNP9Lz

Author: claude

docs/reference/viam-server.md

@@ -291,6 +291,16 @@ Enabling debug level logs will take precedence over all logging configuration se
 | `-webprofile` | Include profiler in HTTP server. |
 | `-webrtc` | Force WebRTC connections instead of direct connections. Default: `true`. |
 
+## Environment variables
+
+You can set the following environment variables to configure `viam-server` behavior without command-line flags:
+
+<!-- prettier-ignore -->
+| Variable | Description |
+| -------- | ----------- |
+| `VIAM_LOGFILE` | Path to a log file. Equivalent to the `-log-file` flag. If both the flag and the variable are set, the flag takes precedence. |
+| `VIAM_NO_WINDOWS_EVENT_LOGGER` | If set to any value, disables writing logs to the Windows Event Logger. Only relevant on Windows. |
+
 ## Install `viam-server` without the web UI
 
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