Add IMGPROXY_GRACEFUL_STOP_TIMEOUT docs

Author: DarthSim

docs/configuration/options.mdx

@@ -35,6 +35,7 @@ echo $(xxd -g 2 -l 64 -p /dev/random | tr -d '\n')
 * [`IMGPROXY_BIND`]: the address and port or Unix socket to listen to. Default: `:8080`
 * [`IMGPROXY_NETWORK`]: the network to use. Known networks are `tcp`, `tcp4`, `tcp6`, `unix`, and `unixpacket`. Default: `tcp`
 * [`IMGPROXY_TIMEOUT`]: _(deprecated alias: [`IMGPROXY_WRITE_TIMEOUT`])_ the maximum duration (in seconds) for processing the response. Default: `10`
+* [`IMGPROXY_GRACEFUL_STOP_TIMEOUT`]: the maximum duration (in seconds) to wait for ongoing requests to finish before shutting down the server. Default: twice the `IMGPROXY_TIMEOUT` value
 * [`IMGPROXY_READ_REQUEST_TIMEOUT`]: _(deprecated alias: [`IMGPROXY_READ_TIMEOUT`])_ the maximum duration (in seconds) for reading the entire incoming HTTP request, including the body. Default: `10`
 * [`IMGPROXY_WRITE_RESPONSE_TIMEOUT`]: the maximum duration (in seconds) for writing the HTTP response body. Default: `10`
 * [`IMGPROXY_KEEP_ALIVE_TIMEOUT`]: the maximum duration (in seconds) to wait for the next request before closing the connection. When set to `0`, keep-alive is disabled. Default: `10`