feat: add OpenAPI preview workflow

[julia-rabello]

Adds a GitHub Actions workflow that posts preview links for changed JSON specs as a PR comment, updating it on each push. Depends on https://github.com/vtex/admin-docs/pull/51

Types of changes

• New content (endpoints, descriptions or fields from scratch)
• Improvement (make an endpoint's title or description even better)
• Spelling and grammar accuracy (self-explanatory)

Changelog

Do not forget to update your changes to our Developer Portal's changelog. Did you create a release note?

• Yes, I already created a release note about this change.
• Not yet, but I'm going to.
• No, it's just a fix.

[github-actions]

Thanks for your contribution. The .json file will be checked now with Spectral.

[vtex-pr-sentinel]

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