Rewrite "Structuring Content"

[wouterj]

This article should give people a decent introduction in the MenuBundle and BlockBundle. At the end of the article, the user should know enough about these bundle to use it in it's CMS.

The Content section is already written. It should be check and some parts should be rewritten or removed. The Block section has not been written yet, you should create a complete new section about that. Main goal: Keep it short and decent.

While rewritting the article, also do the following:

• Admin-related stuff should be removed.
• Configuration sections should also be removed.

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

We need to sync this with cookbook/using_blockbundle_and_contentbundle.rst as well

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.