First pass at extensions docs
Changes from feedback
Added extensions example and consolidated installation into the rest of
the extension docs
Merge pull request #290 from pulp/jdob-docs-extensions

Extension dev guide documentation
Updates to the handler structure based on how the extensions docs came
out
Server plugins introduction
Dev guide docs for type definitions
Merge pull request #296 from pulp/jdob-type-defs

Dev guide docs for the plugin introduction page and type definitions
Overview and descriptor sections.