docs(stablecoins): update VNX website link from vnx.li to vnx.io

[GigaHierz]

What

VNX moved its website from vnx.li to vnx.io. Replaces the three remaining links to the old domain:

• build-on-celo/build-with-local-stablecoin.mdx (VNX row in the issuer table)
• tooling/contracts/stablecoin-contracts.mdx (vCHF and vGBP rows)

No other content changes. Both domains currently return 200, so this is a freshness fix rather than a broken link.

Verification

$ mint broken-links
success no broken links found

$ grep -rn "vnx\.li" . --exclude-dir=node_modules --exclude-dir=.git --exclude-dir=.context
(no output)

$ curl -sI https://vnx.io/ | head -1
HTTP/2 200

🤖 Generated with Claude Code