Activity-type Rings charts

[danwood]

Description

Added new classes RingsChart, RingsChartRow (maybe not needed but I'm following the pattern of the other chart types), and Ring, to implement Activity-type rings charts.

Motivation and Context

I thought it would be useful to implement this as another type of chart provided by the framework.

How Has This Been Tested?

Run on simulator with different sizes.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Non-functional change (Updating Documentation, CI automation, etc..)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

Notes:

I have some other ideas on where to take this, but I wanted to wait a bit for other PRs to come through, and discussion about how to update chart styles so that line widths can be styled, possible display of percentage in the center, etc.

This will need an update to the demo project - see a separate pull request for that.