Add documentation of espnow module and update coap documentation

[geza-pycom]

Same as: #292

[CLAassistant]

All committers have signed the CLA.

[gijsio]

I believe we want this to go into development-publish at some point to be on development.pycom.io, but I thought we discussed it would temporarily go to the branch called idf4 and later merge into an up-to-speed development-publish branch?

lmk

[geza-pycom]

@gijsio : I think right now just let's merge it into idf4 branch, later on it can be decided what to do.

[gijsio]

If we redefine this to go into development-publish we can merge :)
Btw. I noticed your using h4 (####) for the function names, Im trying to convert all API to h3 (###) as that gets picked up by the TOC in the top right corner, thanks!

[peter-pycom]

@gijsio , could you maybe take over this PR and get it ready to merge to devel docs? I can review and we can get this live