Document activites and home/dotsugar

Missing from the basic documentation on using the dev-environment was
an explanation of sugar-build/activities and
sugar-build/home/dotsugar. This patch provides some basic
documentation for these essential subdirs. Also, the link to
dev-environment.md was broken in activity.md. A patch to fix it also
included.
Add documentation for gconf directory

It would be good to point new developers to
sugar-build/home/default/config/gconf as part of the introduction to
the dev environment. Also, removed some stray .s