SEO: Lead with MegaDetector in README and docs

[rmdodhia]

Summary

• Add CITATION.cff — enables GitHub's "Cite this repository" sidebar widget (competitor has this, we don't)
• Restructure README.md — MegaDetector section first, SPARROW Studio second, Model Zoo table, org list expanded (no longer collapsed), announcements moved to bottom
• Rewrite megadetector.md — from ~200 words to ~5,300 words with comprehensive V6 benchmarks, getting started guide, code examples, GPU setup, species classification pipeline, known limitations, and version history

Key changes to review

1. Removed link to agentmorris/MegaDetector in megadetector.md — replaced with pointer to our archive branch. This is the most sensitive change.
2. Org list is now visible, not hidden in a <details> collapse — matches competitor's approach of using social proof prominently.
3. MegaDetector is positioned as the flagship model within SPARROW Studio, not subordinate to PyTorch Wildlife branding.

Not in this PR (repo settings changes to do after merge)

• Update repo description to lead with "MegaDetector"
• Update topics (drop pytorch-wildlife, add wildlife-detection, animal-detection, object-detection, ecology, deep-learning)
• Create MegaDetector-named GitHub Release (megadetector-v6.0)
• Upload new social preview image
• Update Website field to megadetector.ai (only after site is live)

Test plan

• Verify all internal anchor links work when rendered on GitHub
• Confirm org list is accurate and no orgs need to be added/removed
• Review megadetector.md benchmark claims against actual model performance
• Confirm team is aligned on removing the agentmorris/MegaDetector link
• Check that MkDocs site build still works (mkdocs build)

🤖 Generated with Claude Code

[rain-Brian]

Closing this draft — the work here has been absorbed into active PRs:\n- CITATION.cff → #638\n- README restructuring → #638\n- Docs site SEO + naming pivot → #639\n\nThe megadetector.md expansion in this draft is the one piece not yet in a PR; if that content is still needed, it should be a fresh PR off current main.