Update homepage adopters and registry CTA

[nalacateria]

Summary

• Add outcome-first “Why organizations choose Multiformats” cards.
• Add a “growing protocol registry” section with adoption metrics and a CTA to register a multicodec in table.csv.
• Rework the projects section into ecosystem categories covering storage, networking, identity, cryptography, AI, and research users/adopters.
• Keep the existing site style while adding simple responsive cards and table styling.

Validation

• make build

Build succeeds. Hugo still prints the existing warning about both content/index.md and content/_index.md being present.

Screenshots

Full homepage

Registry CTA and ecosystem table