Add Kubit integration documentation

[stefan-enev]

Summary

This PR adds a Kubit integration to Langfuse docs, a dedicated page under Integrations -> Analytics.
The referenced video https://static.langfuse.com/docs-videos/kubit-dashboard.mp4 is not uploaded yet, our understanding is we handle it as part of the PR approval process.

Why this belongs in Langfuse docs

• Kubit allows you to combine Langfuse observation and score events with classical product analytics data and report over both.
• The Kubit integration sends observation and score events to a Kubit endpoint (both legacy and v4 data models are supported)

Validation performed

• Confirmed kubit is reachable in /integrations/kubit and renders correctly
• Confirmed kubit is displayed correctly under Analytics in integrations

Links

• Kubit integration PR: feat(kubit): add Kubit analytics integration langfuse#12798
• Kubit website: http://kubit.ai
• Kubit docs: http://docs.kubit.ai

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[CLAassistant]

All committers have signed the CLA.

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[stefan-enev]

Just added the user.email I was working with to my Github profile so there is only 1 committer. Also signed the CLA so CLA check should pass if re-run.