Update extensions docs for 2.0

I think this page probably needs a few updates: <http://docs.ckan.org/en/latest/extensions.html>

- [x] Links to extensions list on old wiki
- [x] ..and then immediately gives its own list too!
- [x] Installing an extension instructions seem unnecessary put those in each extension's readme (also enabling an extension)
- [ ] Also the background tasks page <http://docs.ckan.org/en/latest/background-tasks.html>, again seems like something better documented per-extensions

So most of this page seems like it's better documented in individual extensions' readme files and should be removed. Maybe just leave some very basic general info about ckan extensions and a link to a new wiki page with a list of them

It doesn't seem lime enough to warrant its own page maybe it should be merged into this page <http://docs.ckan.org/en/latest/writing-extensions.html> then do all extension docs on one page