To signify that this ADC documentation is specific to the WiPy, and to make
way for a standardised ADC documentation.

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".

With this commit there is now only one entry point into the whole
documentation, which describes the general MicroPython language, and then
from there there are links to information about specific platforms/ports.

This commit doesn't change content (almost, it does fix a few internal
links), it just reorganises things.

The WiPy machine.Timer class is very different to the esp8266 and esp32
implementations which are better candidates for a general Timer class.  By
moving the WiPy Timer docs to a completely separate file, under a new name
machine.TimerWiPy, it gives a clean slate to define and write the docs for
a better, general machine.Timer class.  This is with the aim of eventually
providing documentation that does not have conditional parts to it,
conditional on the port.

While the new docs are being defined it makes sense to keep the WiPy docs,
since they describe its behaviour.  Once the new Timer behaviour is defined
the WiPy code can be changed to match it, and then the TimerWiPy docs would
be removed.

This patch also unconditionalizes uos.dupterm(), though exact interface
and semantics is yet to be defined.

For a couple of ports, there was information which directory is set
as current after boot. This information doesn't belong to "uos" module,
and is moved to boards' references (which actually already contained
information on which directory is chosen for boot, even if without
explicit mentioning that it becomes current directory, which is now
done).