Add mock server launch option for OpenAPI example spec

[anshxpress]

This PR adds a mock server link to the README for easier exploration of the OpenAPI example. Devs can now simulate API behavior and test responses instantly—without deploying a backend. It’s handy for frontend prototyping, debugging, and learning how the example contract works interactively.

[github-actions]

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[jayk-u]

Thanks for putting together this suggestion for the readme.
Unfortunately, I won’t be able to approve it, as it leans more toward promotional content than objective information pointing users toward the general concept of mock servers.

That said, we’re always very interested in sharing this kind of solution through complete, in-depth guides that walk users through the process step by step. A great example (on a similar topic) would be this one.

If you ever feel like publishing a guide in that same spirit for the relevant section, we’d be happy to include it! :)