docs: Document esp module for ESP8266.

I document as much as I could guess from experiments and reading the
code for the ``esp`` module for the ESP8266 port of Micropython.

For now the tag has to be set manually with -t option when building,
when we have properly split documentation, there will be a separate
config file for esp8266 with that the tag "port_esp8266" set.

To build use:

make SPHINXOPTS="-t port_esp8266" html