Fix infinite redirect on docs (#1119)

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> [!IMPORTANT]
> Fixes infinite redirect by correcting the redirect path in `docs.yml`.
> 
>   - **Redirect Fix**:
> - Corrects redirect in `docs.yml` from `/docs/baml-nextjs/baml-nextjs`
to `/guides/installation-language/next-js` to fix infinite redirect
issue.
>   - **Misc**:
>     - Removes `slug` from `nextjs.mdx` front matter.
> 
> <sup>This description was created by </sup>[<img alt="Ellipsis"
src="https://img.shields.io/badge/Ellipsis-blue?color=175173">](https://www.ellipsis.dev?ref=BoundaryML%2Fbaml&utm_source=github&utm_medium=referral)<sup>
for 4f1266f. It will automatically
update as commits are pushed.</sup>

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fern/01-guide/08-integrations/nextjs.mdx

@@ -1,6 +1,6 @@
 ---
 title: Next.js Integration
-slug: docs/baml-nextjs/baml-nextjs
+
 ---
 
 BAML can be used with Vercel's AI SDK to stream BAML functions to your UI.

fern/docs.yml

@@ -668,7 +668,7 @@ redirects:
   - source: "/docs/calling-baml/multi-modal"
     destination: "/guide/baml-basics/multi-modal"
   - source: "/docs/baml-nextjs/baml-nextjs"
-    destination: "/docs/baml-nextjs/baml-nextjs"
+    destination: "/guides/installation-language/next-js"
   - source: "/docs/observability/overview"
     destination: "/guide/observability/tracking-usage"
   - source: "/docs/observability/tracing-tagging"