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 Most of the article was written in Feb 2011 and was outdated now.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.
I like what we're saying here, but I'm not sure its as clear as I want. I basically want to say (not word-for-word, just an idea):
>
Unless you're building a bundle you plan to release open source of share between projects, don't follow this guide. Just create services in `services.yml` and be done.

If we talk parameters, I want to keep it short, so that we don't cloud this message:

>
If you *do* want to be able to configure something from within `config.yml`, you can always <link>create a parameter</link> there and then use it somewhere else.