Split "Advanced Container Configuration" article

[wouterj]

This article (which lives in components/dependency_injection) is some sort of dump place for all topics which we couldn't place anywhere else. This doesn't make it easy to find, while some topics in there are really best practises which should be advertised more.

• Three topics ("Marking services as public/private", "Aliasing" and "Decorating Services") are related to the way you make your services open for extensibility. This should be moved into a new article, telling you everything about these concepts and learning you best practises.
• The "Synthetic Services" topic can also be moved to its own chapter, telling you how to inject instances into the container.
• At last, the "Requiring files" topic can be moved to a simple sidebar in the introduction chapter.

[weaverryan]

Good idea - I don't see a downside to this.