Wire middleware sdk docs (`@supabase/middleware`)
https://github.com/supabase/middleware
Preview ref here:
https://docs-git-docs-supabase-middleware-sdk-supabase.vercel.app/docs/reference/middleware/introduction
<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit
* **New Features**
* Added a Middleware SDK reference section to the documentation.
* Added installation guidance for npm, Yarn, pnpm, Deno, and Bun.
* Documented framework-agnostic middleware composition, typed shared
context, ordering, trust, and environment access across supported
runtimes.
* Added Middleware documentation to navigation and search.
* Identified the Middleware SDK as an alpha release.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
