This change is done to reduce ambiguity in some cases;
the previous intention has been to ease navigation when
staying in a particular directory, now it's been changed
in favour of convenient toplevel `git grep' in fact.

Both variants have their pros and cons, I just find myself
leaning to this one by now hence the commit.  Feel free to
provide constructive criticism :)

Some path-related bitrot has also been fixed while at that.

These have been proofread somewhat to correspond
to the current state of affairs; a missing one
was added for fonts feature.

MAIN_GROUPS should align better along with MAIN_PACKAGES
and MAIN_LISTS (even if MAIN_ prefix might be suboptimal
given that these packages are essentially extras within
the particular image).

ToC:
- introduced doc/
- features.in/00example/
- READMEs and TODO for more
- code comments clarified

Some unrelated minor fixes bundled either.