Docs: Improve introduction / quickstart structure

[TomCools]

The introduction to the Solver documentation is somewhat unstructured.

• Introduction already contains "Getting started" code examples.
• "Using the Solver" once again has an overview.
• Modelling and design patterns are not necessarily super Solver Specific and are mentioned both in the "Design Patterns" header AND under "Using Timefold solver -> Modelling planning problems"

Improve the structure so Introduction is just an introduction and links to all other aspects.
It should explain what it is and how to link through to other elements.