docs: add VitePress site for 7.x

[benlesh]

Summary

Applies Jessica Sachs's VitePress documentation rewrite to the current 7.x stable branch.

This preserves Jessica's original commit and authorship from #7591, whose base matches the current 7.x tip, while using the stronger generator and rendering fixes from #7588 as reference.

• builds the API reference from the current 7.x source
• removes committed generated API Markdown and regenerates it during the build
• removes the Nuxt UI coupling that prevented VitePress server rendering
• adds root commands for building and developing the VitePress site
• keeps generated API Markdown and VitePress output out of Git
• leaves runtime source and behavior unchanged

Verification

• npm run docs:build:vitepress
• VitePress client build, server build, and page rendering pass
• TypeDoc reports 0 errors and 246 warnings from legacy JSDoc links and missing relative images

Follow-ups

Legacy API URLs are narrowly exempted from VitePress's dead-link check until versioned deployment and redirect policy is decided. Work for 5.x, 6.x, and 8.x is intentionally deferred.