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:

• [x] #455
• [x] #648
• [x] #456
• [x] #457
• [ ] #534
• [x] https://github.com/frequenz-floss/frequenz-channels-python/issues/177

[matthias-wende-frequenz]

We also need an Introduction to channels.

[llucax]

I guess that should go to the channels repo and we add a link to it in the SDK?