docs: Create a guide on how to use license key validation in a desktop application

[rishi-raj-jain]

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• pick a desktop application starter (ideally react based)
• show how to use license key to make validation requests to the unauthenticated validation endpoint at https://docs.polar.sh/api-reference/customer-portal/license-keys/validate

(only after https://github.com/polarsource/polar/issues/6566 is resolved)