Previously individual ports documented these aspects to varying degrees,
but most of the information is common to all ports.
In particular, this adds a canonical explanation of `boot.py` and
`main.py`.
This work was funded through GitHub Sponsors.
Signed-off-by: Angus Gratton <angus@redyak.com.au>
Replaces "PYB: soft reboot" with "MPY: soft reboot", etc.
Having a consistent prefix across ports reduces the difference between
ports, which is a general goal. And this change won't break pyboard.py
because that tool only looks for "soft reboot".
This patch allows you to stop auto-indent by pressing enter on a second
blank line. Easier than having to use backspace, and prevents new users
from getting stuck in auto-indent mode.
