Add workflow docs

[ethanpalm]

Documentation changes

This PR renames the existing workflow page to "Use cases" and documents the new workflow feature.

Closes https://linear.app/mintlify/issue/DOC-272/docs-for-new-workflows

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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

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Note

Low Risk
Docs-only changes; main risk is broken links or confusing navigation if page paths/titles don’t match the sidebar entries.

Overview
Adds a new agent/use-cases.mdx page with practical examples for using the agent (Slack iteration, PR-based updates, reviews, API automation).

Rewrites agent/workflows.mdx from general examples into Workflows feature documentation, covering plan availability, .mintlify/workflows/*.md file structure, YAML frontmatter fields (name, on, context, automerge), supported triggers (cron/push), and auto-merge behavior.

Updates agent/index.mdx to link to Workflows as another way to run the agent, and updates docs.json navigation to include agent/workflows and the new agent/use-cases page in the Agent section.

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