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.
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.
