Add IMGPROXY_FAIL_ON_DEPRECATION docs

Author: DarthSim

docs/configuration/options.mdx

@@ -714,3 +714,4 @@ We highly recommended reading the [Memory usage tweaks](../memory_usage_tweaks.m
 * [`IMGPROXY_RETURN_ATTACHMENT`]: when `true`, response header `Content-Disposition` will include `attachment`. Default: `false`
 * [`IMGPROXY_HEALTH_CHECK_MESSAGE`]: ((pro)) the content of the health check response. Default: `imgproxy is running`
 * [`IMGPROXY_HEALTH_CHECK_PATH`]: an additional path of the health check. Default: blank
+* [`IMGPROXY_FAIL_ON_DEPRECATION`]: when `true`, imgproxy will exit with a fatal error if a deprecated config option is used. Default: `false`