docs: add langfuse integration to provider list

[jannikmaierhoefer]

This PR adds the Langfuse integration to the provider list.

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
langchain	❌ Failed (Inspect)			Mar 31, 2025 2:25pm

[ccurme]

Error from build:

[errors]: [
        Error: Can't render static file for pathname "/docs/integrations/providers/langfuse/"
            at generateStaticFile (/vercel/path0/docs/node_modules/@docusaurus/core/lib/ssg.js:119:15)
            at runNextTicks (node:internal/process/task_queues:65:5)
            at process.processImmediate (node:internal/timers:459:9)
            at async /vercel/path0/docs/node_modules/p-map/index.js:57:22 {
          [cause]: Error: The `style` prop expects a mapping from style properties to values, not a string. For example, style={{marginRight: spacing + 'em'}} when using JSX.
              at Fa (server.bundle.js:457126:49)
              at K (server.bundle.js:457128:44)
              at Ka (server.bundle.js:457131:184)
              at Pa (server.bundle.js:457139:68)
              at Xc (server.bundle.js:457172:32)
              at Z (server.bundle.js:457177:89)
              at Yc (server.bundle.js:457180:98)
              at $c (server.bundle.js:457179:140)
              at Z (server.bundle.js:457177:345)
              at Xc (server.bundle.js:457172:476)
        }
      ]