Reorganize docs for dev-esp32

[marcelstoer]

Triggered by reading #2694 (comment) again.

Last year @galjonsfigur did a great job reorganizing docs into Lua and C modules (see https://github.com/nodemcu/nodemcu-firmware/blob/dev/mkdocs.yml#L44 ff).
Every Lua module folder has a README that points to the documentation and the respective Lua module documentation. See https://github.com/nodemcu/nodemcu-firmware/tree/dev/lua_modules/email for an example.

We need the same for dev-esp32.