Split docs into siteadmin and pluginwriter guides

* create initial bits for plugin writer's guide
* move siteadmin stuff to site administrator's guide
* rework index.rst to support multiple guides
* tweak some text
* move files into subdirectories

I verified that this still works with html and texinfo build targets.
There's still a lot of work to do, but this is a good start.

docs/source/siteadmin/foreword.rst

@@ -0,0 +1,48 @@
+.. MediaGoblin Documentation
+
+   Written in 2011, 2012 by MediaGoblin contributors
+
+   To the extent possible under law, the author(s) have dedicated all
+   copyright and related and neighboring rights to this software to
+   the public domain worldwide. This software is distributed without
+   any warranty.
+
+   You should have received a copy of the CC0 Public Domain
+   Dedication along with this software. If not, see
+   <http://creativecommons.org/publicdomain/zero/1.0/>.
+
+========
+Foreword
+========
+
+About the Site Administrator's Guide
+====================================
+
+This is the site administrator manual for GNU MediaGoblin.  It covers
+how to set up and configure MediaGoblin and the kind of information
+that someone running MediaGoblin would need to know.
+
+We have other documentation at:
+
+* http://mediagoblin.org/join/ for general "join us" information
+* http://wiki.mediagoblin.org/ for our contributor/developer-focused wiki
+
+
+Improving the Site Administrator's Guide
+========================================
+
+There are a few ways---please pick whichever method is convenient for
+you!
+
+1. Write up a bug report in the bug tracker
+2. Tell someone on IRC ``#mediagoblin`` on Freenode.
+3. Write an email to the devel mailing list.
+
+Information about the bugtracker, IRC and the mailing list is all on
+the `join page`_.
+
+.. _join page: http://mediagoblin.org/join/
+
+Patches are the most helpful, but even feedback on what you think
+could be improved and how to improve it is also helpful.
+