This makes top-level ToC of the pyboard docs consistent with other ports
(consisting of 3 chapters: QuickRef, General Info, and Tutorial).
Also, some other minor tweaks applied, like local ToC for General Info and
headings mentioning pyboard.
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).
