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.