We're migrating to using the cleaner zulip.com domain, which involves
changing all of our links from ReadTheDocs and other places to point
to the cleaner URL.
Punctuate marketing headings with a period. Fix a couple of
title-cased headings to sentense case. Consistently use curly
apostrophes, curly quotation marks, and Unicode ellipses.
Signed-off-by: Anders Kaseorg <anders@zulipchat.com>