feat(showcase): move MDX docs routes + components + libs + content into shell-docs

Extracts everything that exists to render MDX documentation (docs/[[...slug]],
[framework]/[[...slug]], ag-ui/[[...slug]], reference/[...slug]) out of shell
into the new shell-docs package that will serve docs.showcase.copilotkit.ai.

Moves (git mv preserves history):
  - App routes: /docs, /[framework], /ag-ui, /reference
  - Docs-only components: docs-page-view, docs-callout, docs-steps, docs-tabs,
    mdx-components, framework-tabs, framework-selector, sidebar-*, snippet,
    property-reference, router-pivot, stored-framework-highlight, react/*
  - Docs-only libs: lib/docs-render, lib/mdx-registry
  - All content: content/docs, content/ag-ui, content/reference, content/snippets
  - .docs-sync-sha marker (follows the content)

Duplicates into shell-docs (both shells need them):
  - brand-nav, search-modal, search-trigger, copy-button, framework-provider
  - lib/registry.ts, data/registry.json, data/demo-content.json,
    data/search-index.json
  - app/layout.tsx + globals.css + public/{images,logos}

shell-docs gets its own minimal middleware (PostHog-only — no SEO redirect
table, docs host never served legacy URLs). shell keeps seo-redirects.ts
for the legacy-URL migration table; framework-scope protection in its
middleware is now effectively dead but harmless (next.config.ts redirects
fire before middleware ever sees /<framework>/ paths).

InlineDemo updated for cross-host context: 'Open full demo' link points
at the shell host (showcase.copilotkit.ai) since the integration profile
route only exists there.

Author: jpr5