docs: restructure Managed Postgres sidebar and add migration overview

[mintlify]

Summary

Ports ClickHouse/clickhouse-docs#6279 — restructures the Managed Postgres sidebar into four functional groups and adds a new migration decision-guide overview page.

Changes

• New page products/managed-postgres/migrations/overview.mdx — decision guide comparing the four migration paths (ClickPipes, PeerDB, pg_dump/pg_restore, logical replication) across CDC support, where they run, and best-fit scenarios.
• Rewrote products/managed-postgres/navigation.json into four groups: Get started, Use it, Configure, Operate & reference. Data migration now lives under Get started; Extensions becomes a nested group containing pg_clickhouse (cross-listed from integrations/connectors/tools/pg_clickhouse/); Monitoring stays grouped under Operate & reference.

Notes / ambiguity

• clickpipes slug: the source PR links to /cloud/managed-postgres/migrations/clickpipes, but the Mintlify repo has no clickpipes.mdx page in this folder — only clickhouse-cloud.mdx, which describes the same ClickHouse-Cloud-console import wizard. I pointed the new overview's "ClickPipes" links to clickhouse-cloud.mdx. If a dedicated clickpipes page is meant to be added separately, the links should be updated then.
• migrations/faq: the source overview ends with a link to /cloud/managed-postgres/migrations/faq, but no faq.mdx exists under products/managed-postgres/migrations/ in this repo (only the top-level products/managed-postgres/faq.mdx). I dropped the FAQ reference from the overview's "After the migration" section rather than guess at the target. Worth confirming whether a migrations-specific FAQ should be ported.
• pricing: the source sidebar lists a pricing entry under Get started, but products/managed-postgres/pricing.mdx doesn't exist in this repo, so it was omitted from the navigation.
• Mintlify resolves links by file path rather than Docusaurus slug:, so the new overview links use /products/managed-postgres/migrations/... paths.

Context

Source PR: ClickHouse/clickhouse-docs#6279 (restructure-managed-postgres-sidebar).

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Note

Low Risk
Documentation and navigation-only changes with no runtime or security impact.

Overview
Adds a Managed Postgres data migration overview that compares ClickPipes (linked to clickhouse-cloud), PeerDB, pg_dump/pg_restore, and logical replication on CDC, where each runs, and when to use it, plus a post-migration pointer to data validation.

Restructures navigation.json into Get started, Use it, Configure, and Operate & reference: connection and a nested Data migration section (overview first, then existing guides) move under Get started; operational topics consolidate under Use it; Extensions nests pg_clickhouse integration docs under Configure; monitoring, benchmarks, OpenAPI, and FAQ group under Operate & reference (with a small monitoring page order tweak).

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