Update the migration guide for Plugin + App changes.
Update path configuration docs.
First pass at updating the plugin docs.
More tweaks and corrections to the plugin docs.
Update App docs.
Fix mistakes.
Fix suggestsions from ravage84
Merge pull request #764 from cakephp/3.0-plugin-app

3.0 update docs for Plugin + App changes
Resolve folder name conflicts in View, app to App, remove sections of deleted classes.