Add a User Guide

[llucax]

This PR introduces a new User Guide top level section in the documentation explaining how to use the library in a more practical and cohesive way than just reading through the API documentation.

The bulk of the documentation is still hosted in the code itself, as docstrings, so users can still access most of user guide content directly from their IDE.

It also improves the README and the docs home page, adding a quick start section to both.

Fixes #177.

[llucax]

This is based on:

• 244

Only the commits not present in that PR should be reviewed.

[llucax]

You can see the results here: https://llucax.github.io/frequenz-channels-python/v1.0-pre/user-guide/quick-start/

A few comments about stuff I found or thought about while doing this guide:

• Having a higher level documentation in the module documentation makes a lot of sense IMHO, and it is a shame that this documentation is not available in the IDE, so I would like to re-consider the module structure to find a way to expose it. We can discuss elsewhere to avoid polluting this PR too much.
• Having a channel module with a Channel abstract class/protocol, would also allow us to expose docs about general channel concepts that is available in IDEs too.

[llucax]

Rebased

[llucax]

Updated with all the suggestions, except for the ones I left follow-up comments.

[llucax]

Please mark the conversation that you happy with the resolution as resolved.

[llucax]

Fixup done, enabling auto-merge.