Split docs into siteadmin and pluginwriter guides

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.