docs: cohort synchronisation

[Zaimwa9]

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Changes

Contributes to #8420

Adds a Cohort Synchronisation docs category under Third-Party Integrations:

• Overview — concepts: synchronised cohorts become segments, identity matching, environment scoping, synchronisation keys (show-once, revocation, permissions), using and deleting synchronised segments.
• Amplitude — setup guide: create a synchronisation key, add the Flagsmith cohort destination in Amplitude, sync a cohort.

Draft until screenshots are added (placeholders are marked in the pages). A Mixpanel page will follow.

How did you test this code?

npm run build in docs/ passes; prettier clean.

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Project	Deployment	Actions	Updated
docs	Ready	Preview	Sep 1, 2026 3:55pm UTC

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