Add page for PluginAssets shell.

Refs cakephp/cakephp#4976
Adding docs for stopWhen and unfold
reference new file
copy to fr translation
Reorganizing collection docs and adding a list of functions at the top

This document is more similar to a referece page than to a cookbook
Add example for symlinking assets only for specified plugin
Fix indentation
Documenting Collection::sumOf
[fr]follow en
Merge pull request #2148 from cakephp/3.0-plugin-assets

Add page for PluginAssets shell.
Merge pull request #2150 from cakephp/3.0-collection-unfold

3.0 collection unfold