Create "Editing" article

[wouterj]

The book section is currently missing an "Editing" article, which will explain some stuff about the CreateBundle and the Sonata Admin stuff. After reading this article, the user should have enough knowledge about the bundles to create a CMS with it.

All other admin-related stuff in book articles should be stripped out and put in this article. See the getting started section in the old_toc branch for these sections if there are already removed.

For more information see the wiki: https://github.com/symfony-cmf/symfony-cmf/wiki/Documentation-TOC-(DRAFT)#book

If you don't know enough about Restructured Text, you can also provide your PR in Markdown, HTML or a raw office document and I'll convert it into a rst article with your contribution.

[wouterj]

This article could use some texts of cookbook/creating_cms_using_cmf_and_sonata.

There is a link under Adding new pages in book/installation, which should be updated to a link to this article.

[wouterj]

This article should also use some texts of cookbook/installing_configuring_inline_editing

[dbu]

as well as cookbook/creating_cms_using_cmf_and_sonata.rst