docs: add Plans & Activities data format (API.md)

[AhmetBeratKocyigit]

Closes #8166

Describe your changes

This PR adds official documentation for the Plans & Activities data format.
The section describes recommended JSON structures for plans and activities, field definitions, and usage examples.
This improves developer clarity for integrating plan providers into Open MCT.

Changes include:

• Added documentation before Telemetry API section
• Added JSON examples for Plan and Activity objects
• Added definition tables
• Added sample provider usage snippet

No behavior changed — documentation only.

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All Submissions

• Followed the Contributing Guidelines
• Checked for existing similar PRs
• This does NOT require special release note mention (docs-only change)

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Author Checklist

• Addresses original issue
• Markdown renders correctly
• Added appropriate type:docs label
• Milestone assigned (maintainer may set later)
• No functional changes → no tests required

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Reviewer Checklist

• Changes address issue
• Verified viewable output
• No breaking behavior introduced
• Documentation formatting appropriate