Start the ORM intro chapter.
Add missing docs for changing the entity a table uses.
Finish off ORM intro.
Add stub from ORM docs.
Change title.
Fix broken links and limit toc depth.

Entity link was broken and the toc depth was unwieldy.
Expand validation docs.

Add docs around validation providers and which providers are registered
by default. Add skeleton to validation docs.
Correct grammar and tone in table object docs.
Start validation docs.
Merge pull request #966 from cakephp/3.0-orm-intro

3.0 - Add an intro chapter for the ORM and start validation chapter.