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 )