Integrate the "Create your own framework" tutorial into the Symfony official documentation

The "Create your own framework" tutorial I've published on my blog at the beginning of 2012 is still very popular based on the traffic it still receives. Problem is that those articles are not up to date anymore. The RST files on my Github account have been updated though.

So, as there is a lot of interests, and because the up-to-date version is not easily browseable, I propose to integrate the tutorial into the Symfony official documentation and website.

That will also let more people have a look at it and perhaps improve the documentation over time.

``let's`` is still a first person perspective, which is avoided in the doc. However, the doc team is better than me at rewording such sentences to eliminate them.