Unify specification styling and add signup to every header

[callumalpass]

Summary

• Import normative spec content into the shared Astro documentation shell, including the v0.2 archive
• Use the SDK fonts, theme controls, navigation and responsive layout
• Add a right-aligned Sign up link to the shared header on every page
• Test spec imports in CI and production builds

Validation

• Astro checks, build and local link checks passed
• Spec importer regression tests passed
• Verified all 30 pages include signup; browser checks at 320, 390 and 1440px
• Deployed and verified on https://mdbase-dev.pages.dev; approved for production