fix: replace/remove dead PSU personal website links

[davidheyman]

Summary

• Replace Cynthia Brewer profile links (personal.psu.edu/cab38/) with her current PSU faculty page (geog.psu.edu/directory/cynthia-brewer)
• Replace academic reference links (ColorSch, Brewer_pubs) with Wayback Machine archives
• Replace ColorBrewer 1.0 link with Wayback Machine archive (in index.html and flash/index.php)
• Remove dead ColorBrewer_updates.html nav link in index.html (keep "updates" text)
• Point "Further Reading" in credits to the GitHub repo instead of dead updates page
• Replace dead Excel download link with a note that it's no longer available
• Remove dead geovista.psu.edu anchor, keeping the <img> tag

Closes #43

Test plan

• Verify all replaced links resolve (Wayback Machine archives, geog.psu.edu faculty page, GitHub repo)
• Verify nav bar still displays correctly with plain "updates" text
• Verify credits page layout is intact after geovista anchor removal
• Verify downloads page displays the unavailability note properly

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