ESP32 Forum

This one:

https://github.com/espressif/arduino-es ... /WebServer

I looked for a while and it's not included in the official doc. Just wish to read a doc alongside the example code for completeness. Thanks.

There is no official documentation of the library. You can see the headers at https://github.com/espressif/arduino-es ... ebServer.h, which shows you the available methods and parameters. I don't think it would be very useful for learning WebServer, since html depends as much on the browser as the server. It would end up being more like a man file (eg ESPAsyncWebServer).
If you want a tutorial, which shows how the server and browser interact, and explains the basics of what WebServer is doing, google knows lots of them. https://lastminuteengineers.com/creatin ... duino-ide/ looks like a good one.
If you have specific questions about advanced topics, ask away.