Remove non-standard installation instructions.

Suggesting people monkey around with the installation paths will only
cause issues down the road.
Start updating the controller docs.
Fix some duplication in request/response docs.
Update response docs.

Remove API section and put method links inline.
Add view variables section.
Remove pointless docs.

This should be covered in the API not the book.
Make corrections from feedback.
Merge pull request #1523 from cakephp/3.0-cleanup

3.0 cleanup