Migrate RTD/MkDocs build to Python 3

[marcelstoer]

I wanted to enable the RTD docs build for @jmattsson's dev-esp32-idf4 branch so we can announce that branch in the docs early. The build failed with "Could not find a version that satisfies the requirement ziglang>=0.8.0". Checking the RTD logs made me realize we're still using Python 2 to build the docs.

Upgrading to Python 3 (in the RTD settings) worked but the build failed again but for a different reason. Maybe it's because the MkDocs version we (or RTD?) use is ancient as well.

[jmattsson]

Maybe have a look at https://github.com/readthedocs/readthedocs.org/issues/5474? It looks like the same issue we're hitting, and there seems to be solution(s) there :)

[marcelstoer]

WIP https://nodemcu.readthedocs.io/en/update-rtd-mkdocs/

A couple of changes were required.

[marcelstoer]

• [x] Implemented in the dev-esp32-idf4 branch, which will land on dev-esp32 eventually, with #3457.
• [x] Implemented in the dev branch, which will land on release eventually, with #3458.

=> done