Rewrite "Static Content"

[wouterj]

The article is just moved from the getting started section to the book section. This article should be a very decent introduction to the ContentBundle. At the end of the article, the user should know enough about the bundle to use it in it's CMS.

While rewritting the article, also do the following:

• Admin-related stuff should be removed.
• Configuration sections should also be removed.
• don't forget that MultilangStaticContent was dropped in favor of StaticContent

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.