Added espnow module with documentation.

[jmattsson]

Fixes #2924, at least for the ESP32 range.

• [x] This PR is for the dev branch rather than for the release branch.
• [x] This PR is compliant with the other contributing guidelines as well (if not, please describe why).
• [x] I have thoroughly tested my contribution.
• [x] The code changes are reflected in the documentation at docs/*.

This is a pretty straight-forward module implementation of the regular ESP-NOW API. Tested on ESP32-C3, but should be board agnostic.

@pjsg, quick review if you have the time, please? @marcelstoer, could you please check I did the docs properly? It's been a while since I was active...

[marcelstoer]

Here's what I found, pls verify. 0001-Docs-clean-up.patch

[jmattsson]

Thanks Marcel!

I'll keep the small non-espnow updates out of this PR, and will probably just push them directly later 👍

[jmattsson]

Alright, I'm making an executive decision and will merge this. I'm working on getting support for the C6 and H2 into the build, and this will collide with that so I'd rather just land this now and make life easier for everyone. Well, myself at least :D You may yell at me later if you object!

[poorandunlucky]

espnow works on esp8266, too...