Docs: Update info on python.microbit.org

[microbit-mark]

Attempts to ease some of the confusion we have on support tickets where people are trying to open MicroPython projects in MakeCode.

• Change to make the Python Editor the default way of programming
• Mention that the code won't work in MakeCode

[microbit-carlos]

Superseded by https://github.com/bbcmicrobit/micropython/pull/732.