v1.8.1 to add examples AsyncWebServer_MQTT, AsyncWebServer_MQTT_RP2040W and improve README.md

[khoih-prog]

Releases v1.8.1

1. Add AsyncWebServer_MQTT for ESP32 example to demo how to use AsyncWebServer and AsyncMQTT_Generic together
2. Add AsyncWebServer_MQTT_RP2040W example to demo how to use AsyncWebServer_RP2040W and AsyncMQTT_Generic together
3. Improve README.md so that links can be used in other sites, such as PIO

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This discussion was created from the release v1.8.1 to add examples AsyncWebServer_MQTT, AsyncWebServer_MQTT_RP2040W and improve `README.md`.