Demote old guide

It's really out of date. It's also written very much in first person,
which was appropriate when it was written, but isn't apropriate for
use as the ZODB guide.  It doesn't help that the author no lomger uses
ZODB and even wrote an alternative.  Fixing all of this woule entail a
rewrite, and then the licence.  It makes more sense to make this an
article and start a new guide, which I'm doing.

Fix small grammar error

logo

new overview

Mail zodb-dev@zope.org was broken.

"zodb" was missing in the link of the html page.

Update multi-zodb-gc.rst

Small spelling/grammar changes.

typo: inappropriate comma removed

Fixed small grammar errors

their -> they're, it's -> its

Update index.rst (fix broken link)

Fix broken link (https://bugs.launchpad.net/zodb/+bug/1287046)

Improve the README

tutorial enhancements