[New TOC] Turned "getting started" into "book"

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | all
| Fixed tickets | -

This is a first step into the new documentation structure. I renamed "getting_started" to "book", renamed some articles, and quickly rewrote "simplecms" and "installation" so it makes sense.

I'll create tickets for the other articles, so someone can rewrite those, and I create tickets for the missing book articles.