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

[rishi-raj-jain]

as a continuation of #6571

• pick a starter desktop application starter (ideally a react based)
• show how we require the activation_id to be stored locally on the user's device to make sure that it's passed in the unauthenticated endpoint at https://docs.polar.sh/api-reference/customer-portal/license-keys/activate

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