Split the new associations chapter out.

Having a separate associations chapter will help keep the TOC for models
more reader friendly and make for smaller chapters overall.
Split out delete docs.
Move saving documentation into a separate chapter.
Fix markup errors.
Fix title levels.
Move validation docs into one place.
Feedback from code review.
update include link
Correct Collection::reduce() example argument ordering
Merge pull request #2113 from dakota/3.0

Correct Collection::reduce() example argument ordering
Remove stray char.
Remove tmp comment.
Merge branch '3.0' into orm-reorg

Conflicts:
	en/orm/table-objects.rst
Merge pull request #2099 from cakephp/orm-reorg

ORM documentation re-structuring
[fr] follow fe79795f and #2113
[fr] review View chapter
[fr] typos
[fr] follow #2099