Add references to other docs in tutorials

[matthias-wende-frequenz]

1. To the minimum requirements (or to the whole installation section, as pointed out in a follow-up comment)
2. To some docs explaining how to use the sandbox (and eventually how to setup or buy a plan or something to get a "microgrid system" working)
3. An introduction to microgrid concepts (Basic microgrid concepts and component graph #456)
4. Channels introduction (Write introductory documentation frequenz-channels-python#177)

Originally posted by @llucax in #656 (comment)

[matthias-wende-frequenz]

1. we should refer to the README.
2. can be addressed when the sandbox api is finished.

[matthias-wende-frequenz]

@llucax are you working on 4)?

[llucax]

@llucax are you working on 4)?

Yes