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 Most of the article was written in Feb 2011 and was outdated now.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.
Instead of explaining it, why not walk them through a real example, and show them `config.yml` as quickly as possible. Something like:

>
Suppose you're building a bundle that fetches tweets from Twitter. To do this, your bundle needs to be configured with a Twitter `client_id` and `client_secret`, so that it can make the API requests.