docs: add footer-links configuration documentation

[devin-ai-integration]

docs: add footer-links configuration documentation

Summary

Added comprehensive documentation for the footer-links configuration option in the what-is-docs-yml.mdx reference page. This documents an existing feature that was previously undocumented, allowing Fern users to add social media and community links to their documentation footer.

The new section includes:

• A brief explanation of the feature's purpose
• A complete YAML example showing all 12 supported link types
• Individual parameter documentation for each link option (GitHub, Slack, X/Twitter, LinkedIn, YouTube, Instagram, Facebook, Discord, Hacker News, Medium, and Website)

This is the first of three documentation PRs based on a docs.yml schema review, documenting previously undocumented configuration options.

Review & Testing Checklist for Human

• Preview the documentation page to verify the new section renders correctly with proper formatting
• Spot-check 2-3 of the documented options (e.g., github, slack, x) against the docs-yml.schema.json to confirm accuracy
• Verify the section placement (after navbar-links, before background-image) makes logical sense in the document flow

Notes

• This change is documentation-only and does not modify any code or configuration
• All documented options were verified against the schema definition at the time of writing
• The documentation style (ParamField components, YAML code blocks) matches existing patterns in the file
• Requested by danny@buildwithfern.com (@dannysheridan)
• Devin session: https://app.devin.ai/sessions/6f152d767e8e4d6aa28d737373aa47f9

[devin-ai-integration]

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[github-actions]

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[github-actions]

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