Fix stale vcpkg manifest mode link

[GoXLd]

Summary

The vcpkg example still links to the legacy ReadTheDocs page that was
replaced in BUILDING.md by #335. The old URL currently returns a
Cloudflare verification page with HTTP 429 instead of the documentation.

Update the example to use the same current Microsoft Learn manifest mode
page as BUILDING.md.

Verification

• Confirmed the previous URL returns HTTP 429 and a Cloudflare challenge
• Confirmed the Microsoft Learn URL returns HTTP 200 without redirects
• Confirmed the replacement page documents manifest mode and vcpkg.json
• Confirmed no references to the previous URL remain
• git diff --check

Build not run because this changes only a documentation URL.

[zpostfacto]

Thanks!