docs(morph): update published package guidance

[alowpoly]

Summary

• remove obsolete prerelease and registry-availability language now that PolyCSS Morph is published
• add the Morph npm install command to the root, package, and website guidance
• align triangle-rendering copy with the current Chromium, Firefox, and WebKit paths

Validation

• pnpm --filter @layoutit/polycss-morph test:package
• pnpm test
• pnpm build