python.microbit.org is the micro:bit Foundation's deployment of the open source web Python Editor, which was developed by @ntoll with the help of the global Python community. It was created for the original BBC micro:bit project in the UK and is now living a life of it's own as an awesome projet. The source is available at https://github.com/bbcmicrobit/PythonEditor.
The Python editor relies on the MicroPython project for the code that actually runs on the micro:bit.
The documentation can be found at the Python documentation site