[WIP] Documentation update

[ioleo]

This PR is a work in progress. The goal is to completely rewrite all the documentation.

• update README
• bootstrap new documentation structure
• complete Installation section
• complete Getting started section
• complete How does this work? section
• complete Configuration section
• complete Customization section
• complete Cookbook section
• create "Showcase" page with screenshots of demo project on AdminLTE v2 theme
• remember to fix Fix doc #120

Quick draft:

1] Installation

Bundle installation notes.

2] Getting started

Quick guide how to get started.

• assumed knowledge (how to create bundles, models/entities, links to symfony2 doc)
• create admin generator in existing bundle
• create new bundle with admin generator

3] How does this work?

• admingenerator route
• generated "base" controllers and templates
• basic configuration to control built-in features
• extending base files for advanced customization

4] Configuration

• configuration reference for all builders List/Show/New/Edit/Actions/Excel

5] Customization

• how to customize forms/filters/view templates

6] Cookbook

• recipies for common problems, eg. use FormExtensionsBundle, use KnpMenuBundle, non-standard namespace, etc

Fixes: #10, #11, #12, #13, #14, #15, #16, #17, #18, #21.

[bobvandevijver]

👍 for updating the docs!

[bobvandevijver]

Closed in favor of #223.

[ioleo]

@bobvandevijver can the branch be safely deleted?

[bobvandevijver]

Lets keep it for now, I've added it to the to-do list of #223.