Documentation resources for the various microprocessor platforms such as Arduino, NodeMCU & ESP, Raspberry Pi, and many others
Official Arduino learning site.
A great starting point for those who want to learn how to use and program the NodeMCU microcontroller.
An in-depth guide to the ESP8266 microcontroller. This guide does expect the user understand some basic microcontroller concepts and therefore isn't a starting point for learning about the ESP8266 from scratch. The guide is very useful for those who would like to transistion to the ESP8266 from another platform like the Arduino.