Add basic getting started documentation

[llucax]

What's needed?

We need new users to have enough information to get started using the SDK.

Proposed solution

Add at least the following information:

• Installation instructions #455
• Write a small introduction to actors #648
• Basic microgrid concepts and component graph #456
• A step-by-step tutorial on how to build a full app #457
• Add documentation section about how the data pipeline works #534
• Write introductory documentation frequenz-channels-python#177