Quick proof of behaviors docs.
Quick proof of the component docs.

* Remove wrong information and update code examples.
Update migration guide for cakephp/cakephp#4094
Restructure the TOC on FormHelper docs.

Include more headers so the page contents and page are easier to scan.
Update helper docs.

* Add headings for quicker navigation.
* Remove docs for goofy methods. HtmlHelper has lots of goofy methods
  that we don't need to promote.
* Fix missing namespace flags.
* Remove parameter docs. The API docs should be the source of truth on
  this.
Update docs based on review feedback on cakephp/docs#1530
Merge pull request #1530 from cakephp/3.0-core-libraries-proof

3.0 core libraries proof