docs: cover more platforms in pg_cron section

[NikolayS]

Extends the "Enabling pg_cron on managed platforms" section in docs/installation.md to cover newly-added platforms. Each entry uses the documented enablement mechanism plus the shared create extension/pgque.start() pattern. Where the official docs do not publish an exact pg_cron version (Crunchy Bridge, Tiger Data, IBM, Oracle OCI), no version is asserted in prose.

Managed clouds:

• Crunchy Bridge — preloadable, create extension pg_cron;
• Tiger Data (Tiger Cloud) — tsdbadmin runs create extension
• ClickHouse Cloud (Managed Postgres) — create extension pg_cron; (1.6)
• IBM Cloud Databases for PostgreSQL — create extension + cron.schedule()
• Oracle OCI Database with PostgreSQL — enable via custom config / Extension Manager, then create extension
• Ubicloud Managed PostgreSQL — create extension pg_cron; (1.6)

Kubernetes operators:

• Zalando postgres-operator — add pg_cron to shared_preload_libraries in the cluster manifest, then create extension

Source of truth: evidence comments on issue #270.

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