Documentation: Add PostURL component docs

[kellenmace]

What? & Why?

Addresses one item in #60358

Adding documentation to existing editor components can help with any of the following:

• encourages knowledge sharing and quicker onboarding for future devs
• supports maintenance and troubleshooting
• mitigates risk

How?

Add JSDocs formatted doc blocks to the existing PostURL component.

Testing Instructions

Add a JSDoc comment to the PostURL component and run npm run docs:build to populate the README.md file with the newly added documentation.

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