Create Avalonia topic

[worldbeater]

• I followed the contributing guidelines.
• I have no affiliation with the project I am suggesting (as a maintainer, creator, contractor, or employee).

Which change are you proposing?

• Suggesting edits to an existing topic or collection
• Curating a new topic or collection

---

Curating a new topic or collection

• I've formatted my changes as a new folder directory, named for the topic or collection as it appears in the URL on GitHub (e.g. https://github.com/topics/avalonia)
• My folder contains a avalonia.png image and index.md
• All required fields in my index.md conform to the Style Guide and API docs.

[kenyonj]

Looks great. Thank you for the contribution!