Minor documentation tweaks for GeoDjango, including addition of link to main...

Minor documentation tweaks for GeoDjango, including addition of link to main index, and removal of unsupported `:download:` directives.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@12942 bcc190cf-cafb-0310-a4f2-bffc1f526a37