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.
The idea is to allow to define a kind of "macros" for repeatitive text,
so all occurrances can be updated in one place. Unfortunately, RST doesn't
support replacements with arguments, which limits usefulness of them and
should be taken into account.
We have enough terms or references throughout the docs which may be not
immediately clear or have some important nuances. Referencing terms in
gloassary is the best way to deal with that.
