Update Preview Mode Documentation

Added more code examples and some screenshots, also
cleared up text a bit.
Fixed typo
Added links to new action_xyz docs in index.rst
Fixed formatting in form_types.rst
+ added stub document for export documentation and linked it from index.rst
Fixed formating in translation.rst
Removed reference to bc strategy
Added example to the paragraph at the end
( Closes #1534 )
getting started
Create recipe_image_previews.rst

Created cookbook entry about how to add image previews to edit forms
Create recipe_file_uploads.rst
Merge pull request #1564 from caponica/recipes-01

Created recipes for file uploads and image previews
Merge pull request #1563 from tiagojsag/getting_started

Getting started
Merge pull request #1562 from caponica/docs-20130810

Small updates to documentation
Merge pull request #1558 from koyaan/doc_preview_mode

Update Preview Mode Documentation ( Issue #1539 )
Added links to two new cookbook recipes
Standardised link formats, moving html links to references instead of inline
Added link to Image Preview cookbook entry from form_help_messages
removed the mention of sonata_type_translatable_choice being deprecated
Added mention that excludes takes priority over other settings
Changed definition formatting to use a definition list
Change Admin::getClass logic to use subclass/subject class

Fix Issue #1466 create/update routing problem in form creation, when id is 0
Fix in part Issue sonata-project/SonataDoctrineORMAdminBundle#250
Fix Issue #924 Broken field description when using subclasses
Merge pull request #1509 from tashanta/fixSubclass

Fix issue  sonata-project/SonataDoctrineORMAdminBundle#250