update help page with feature flags

[microbit-mark]

• pass feature flags to help page in order to show/hide features
• pass MicroPython version from editor to docs
• replace static images of buttons in help with css/fa
• some more readability edits

[microbit-carlos]

Hi Mark,
It all looks good and I think @microbit-rosslowe already reviewed this in the python-editor repo.
One last request I hope you don't mind. Can we update the header image to this one?
Since by default some of the buttons will not be shown (and all the icons are then showed and described below) I think it's probably better to provide a screenshot of the default header, rather than the full one.
header.png.zip

[microbit-mark]

hey @microbit-carlos. That makes sense. Ack. and done

[microbit-carlos]

Great, thank you Mark!