Add filename in openapi-schema frontmatter docs

[lawreka]

Documentation changes

Brief description of what's being updated

Describes changes added in https://github.com/mintlify/mint/pull/4019

Closes

n/a

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

[ethanpalm]

lgtm