Update migration guide for submit() and inputs().
Add/update docs for inputs().
Re-organize and add new titles.

Add titles to break up blobs of text, and make scanning easier.
Fix broken links.
Update migration guide for Helper method removal.
Update Helper docs for method removal.
Fixup issues found in review.
Merge pull request #1131 from cakephp/3.0-form-4

3.0 form 4