docs/esp8266/general: Start "General information" for esp8266.

docs/esp8266/general.rst

+General information about the ESP8266 port
+==========================================
+
+ESP8266 is a popular WiFi-enabled System-on-Chip (SoC) by Espressif Systems.
+
+Multitude of boards
+-------------------
+
+There are multitude of modules and boards from different sources which carry
+ESP8266 chip. MicroPython tries to provide a generic port which would run on
+as many boards/modules as possible, but there may be limitations. Adafruit
+Feather HUZZAH board is taken as a reference board for the port (for example,
+testing is performed on it).
+
+WebREPL
+-------
+
+TBF