Clarify multiple API specification files

[ethanpalm]

Documentation changes

This PR clarifies the use of multiple API specification files.

Closes https://linear.app/mintlify/issue/DOC-226/editor-how-to-add-multiple-jsonyaml-files

---

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

---

Note

Enhances documentation for configuring API specs and auto-generated pages.

• AsyncAPI setup: Adds examples for referencing multiple asyncapi files in navigation and notes each file generates its own channel pages
• OpenAPI setup: Updates to support one or more openapi files with CodeGroup examples for single and multiple specifications; adds guidance on hosting/adding specs
• Global settings (docs.json): Fixes expander casing to api/openapi/asyncapi and adds clear examples for openapi/asyncapi as single file, array of files, or directory objects
• Editor configurations: Minor label cleanup for OpenAPI specs

^{Written by Cursor Bugbot for commit 22c8d43. This will update automatically on new commits. Configure here.}