licensing: add documentation on how to generate a license key for testing and debugging

Created by: miveronese

Part of #40064 (closed)

This PR updates the docs with instructions for devs on how to generate a license key for testing and debugging locally.

Test plan

Manually tested locally by checking the pages

• http://localhost:5080/dev#plans-and-licenses to confirm that the new section "#Plans and licenses" was successfully created.
• http://localhost:5080/dev/how-to/generate_license_key_for_testing to confirm that new page was created.