add Polar to the open source projects section

[mikolysz]

Motivation

I think the project belongs in this repo because:

• It is one of a very few somewhat large-scale open source projects that uses Fast API.
• For people who want to see how actual, real-world Fast API apps are structured, what patterns they use and how things are done commercially, in environments where the framework is being used for commercial purposes, it's an invaluable learning resource.
• Despite being the codebase for a for-profit company, the project is Apache licensed, which allows developers to reuse code fragments in their own apps.