Improve extensions documentation

[rgrp]

Current extensions documentation needs some work: ​http://docs.ckan.org/en/latest/plugins.html

• Queue extension section may now be out of date (?)
• Think about how it integrates with ​https://github.com/okfn/ckanext-example (especially tutorial and example extension)
• Document all plugin points (auto-extract from CKAN source??)

[rgrp]

@dread: re-assigning to you (at least to review). Would be really good to have this closed out asap.

[dread]

Have not looked at this yet. Bumping to this sprint.