Rewrote Extension & Configuration docs

The "How to expose semantic configuration for a bundle" article always was a bit of a mess. It tried to explain how to use Extension classes to load service definitions and how to provide configuration options for a bundle. At the same time it showed 2 ways to achieve this goal: a simple way and the official way. This resulted in an article that wasn't easy to follow and did not really learn anything about Extensions and Configurations.

So I decided to rewrite the complete article into 2 seperate articles: One about the Extension and one about the Configuration.

While I was doing that, I discovered it was time that we rewrote this thing. Most of the article was written in Feb 2011 and was outdated now.

I've also added a section about XML configuration.

This is a first sketch, I haven't checked my english/grammar/typos, but it's ready to catch the first comments!

| Q   | A
| --- | ---
| Doc fix? | yes
| New docs? | yes
| Applies to | 2.3+
| Fixed tickets | -

Side-note: I have discussed with myself to move the article to the components docs, because most of it doesn't rely on the Symfony framework. However, since the conventions do, I decided to keep it in the cookbook to make it easier to follow.
"very" is an adverb that says something about the "friendly" abverb. So the "friendly" word is extended using "very". If a word can be extended, it isn't closed-class.

I may be wrong, let's see what @weaverryan thinks