docs: add Managed Postgres RBAC page

[mintlify]

Summary

Ports the new Managed Postgres RBAC documentation from ClickHouse/clickhouse-docs#6341.

Changes

• Added products/managed-postgres/rbac.mdx (slug /cloud/managed-postgres/rbac) with Mintlify transforms: BetaBadge/Image snippet imports, IdealImage → <Image> snippet, :::note → <Note>, frontmatter sidebar_label → sidebarTitle.
• Added three screenshots under images/managed-postgres/rbac/.
• Inserted the page in products/managed-postgres/navigation.json under "Features" after security, matching the upstream sidebars.js ordering.

Context

Upstream PR: ClickHouse/clickhouse-docs#6341 (branch pg-rbac).

Notes

• The upstream <BetaBadge> uses link/galaxyTrack/galaxyEvent props that the Mintlify BetaBadge snippet does not accept; those tracking props were dropped. Flagging in case the Mintlify snippet should be extended to support them.

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Note

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