[esp32_improv] adding next_url variable documentation (#5369)

Author: amankrokx

content/components/esp32_improv.md

@@ -40,6 +40,8 @@ esp32_improv:
 - **identify_duration** (*Optional*, [Time](#config-time)): The amount of time to identify for. Defaults to `10s`.
 - **wifi_timeout** (*Optional*, [Time](#config-time)): The amount of time to wait before starting the Improv service
   after Wi-Fi is no longer connected. Defaults to `1min`.
+- **next_url** (*Optional*, string): The URL to open after provisioning is complete. Defaults to
+  `https://my.home-assistant.io/redirect/config_flow_start?domain=esphome`.
 
 - **on_start** (*Optional*, [Automation](#automation)): An action to be performed when Improv is waiting for
   authorization and/or upon authorization. See [`on_start`](#improv-on_start).