docs: Add a "Reset and Boot Sequence" reference page.

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>

docs/wipy/tutorial/reset.rst

@@ -16,6 +16,8 @@ There are soft resets and hard resets.
         import machine
         machine.reset()
 
+For more information, see :doc:`/reference/reset_boot`.
+
 Safe boot
 ---------