Add RectPainter sample

[david-c14]

What does the pull request do?

Adds a new sample. A simple graphics editing tool as a demonstration of live interaction between mouse, offscreen image, and rendering

Scope of this PR:

• [ ] fix or update to an existing sample
• [x] add a new sample

Checklist

If this is a new Sample

• [x] Added a ReadMe-file
• [x] Updated the landing page
• [ ] Consider submitting a PR to https://github.com/AvaloniaUI/Documentation with a link to your sample in the related documentation page

In any case

• [x] Spell-checking done
• [x] Checked if all hyperlinks work
• [x] Checked if all images are visible

Fixed issues

[david-c14]

@dotnet-policy-service agree

[david-c14]

I need to implement a graphics editing tool in AvaloniaUI, and came looking for a sample to get me started. But I couldn't see anything particularly close. So I built a simple test example to try out what I was doing, and having got it to work, I thought I should submit it back as a sample to fill the gap.

[maxkatz6]

Thank you! Sample like this is what I wanted to add for a while now. I am going to review on this week.