Openapi nav updates

[mycho2003]

Documentation changes

This PR updates our docs on creating API pages with instructions for the more flexible use of OpenAPI specs.

• Conceptual and procedural information on configuring navigation for endpoint pages generated from OpenAPI specs
• Guide on migrating from individual MDX pages for endpoints to using an OpenAPI spec and the navigation in docs.json

Closes https://linear.app/mintlify/issue/DOC-116/openapi-docs-granular-control

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For Reviewers

When reviewing documentation PRs, please consider:

✅ Technical accuracy

• Code examples work as written
• Commands and configurations are correct
• Links resolve to the right destinations
• Prerequisites and requirements are accurate

✅ Clarity and completeness

• Instructions are clear and easy to follow
• Steps are in logical order
• Nothing important is missing
• Examples help illustrate the concepts

✅ User experience

• A new user could follow these docs successfully
• Common gotchas or edge cases are addressed
• Error messages or troubleshooting guidance is helpful

[mycho2003]

looks great! just one small detail...

[ethanpalm]

Great collaboration ✨

lgtm!