[Improvement]: Make JWT examples more practical

[docs-bot]

Is this issue for a product update or release that is not yet available to customers?

No? Then please continue. Thank you!

Describe the desired documentation updates and the problem they solve for customers.

We should make the examples in Generating a JSON Web Token (JWT) for a GitHub App more practical. Instead of printing the output, we should show how the generated token can be used in an API request.

Which existing articles, if any, are affected?

Generating a JSON Web Token (JWT) for a GitHub App

Who does this affect?

GitHub App developers

What is the impact to users?

easier time building a GitHub App

We welcome contributions, are you able to help?

docs-team or OS contributors

[welcome[bot]]

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