Copy what's in Briefcase's docs

[ahonnecke]

As for toga-chart's documentation, it probably wouldn't hurt to copy what's in Briefcase's docs....although, it looks like the docs here could use a little TLC; for instance, I see a reference to Python 3.5 and it isn't using the [furo](https://github.com/pradyunsg/furo) theme.

Originally posted by @rmartin16 in #61 (comment)

[freakboy3742]

Toga-chart barely has any documentation, so... yes, there are updates required. :-)

However, this issue has been raised against the Toga' repository, and Toga's docs do have much of the same contributor documentation that Briefcase has, customised for Toga's needs.

[freakboy3742]

To clarify the scope for this as a ticket - the aim if this ticket is "write initial docs for toga-chart". The broad structure of docs for a BeeWare project is always the same - "tutorial, howto, topics, reference, about" - and toga-chart should follow that pattern. This includes:

• Use of the beeware theme additions to Furo
• A Read The Docs configuration
• tox configurations that have docs-live, docs-lint, and docs targets for local testing

In terms of content - it's OK for there to be lots of gaps in the first pass. We don't need to have any actual howtos or topics in the first pass. The FAQ can be very bare bones with some basic "positioning" questions - "What is toga-chart" is possibly all we need. At least one link to the matplotlib docs (probably on the topic guide page?) would be called for.

However, we do need a basic tutorial. It doesn't have to be very detailed, or document how to use matplotlib in detail - "add a chart as the only widget in an app" is all you need. Doing a line-by-line walkthrough of the example app in the toga-chart repo is probably sufficient.