Add TrayIcon sample app

[logiclrd]

What does the pull request do?

Adds a sample application demonstrating the use of <TrayIcon> in App.axaml, including an associated menu.

TrayIcon.csproj optimistically references as-yet-nonexistent version 11.2.0 of Avalonia, in the hopes that it will include the bug fix for crashing when activating tray icon menu items.

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/avalonia-docs 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