docs: add fern docs md check command to CLI reference

[devin-ai-integration]

Summary

Documents the new fern docs md check CLI command (added in fern-api/fern@a8b3f2e, CLI v4.43.0) on the CLI reference commands page.

Changes:

• Added a row to the Documentation commands table linking to the new section
• Added an accordion section describing the command's usage, success output, and error output format

Updates since last revision

• Clarified that the command validates MDX files across all navigation configuration files (including versioned configs and product-specific YAML files), not just docs.yml

Review & Testing Checklist for Human

• Verify the example error output format matches what the actual CLI produces (the examples are representative, not copied from real output — run fern docs md check on a project with a syntax error to compare)
• Confirm the claim that the command covers versioned configuration files and product-specific YAML files is accurate (based on the parseDocsConfiguration + getAllPages code path)
• Check that the anchor link #fern-docs-md-check resolves correctly on the preview page

Notes

• The changelog entry for this command already exists on main at cli-changelog/2026-03-22.mdx
• The command takes no options, so the docs section is intentionally simple

Link to Devin session: https://app.devin.ai/sessions/0b40cea8ae384c72a09b94bc27750ca6

[devin-ai-integration]

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[fern-api]

Requested by: ryan
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[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1774222341-docs-md-check-command.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/cli-api-reference/cli-reference/commands