feat(doc-1771): add CDC cookbooks for MySQL, PostgreSQL, MongoDB, SQL Server, TigerBeetle, and GCP Spanner

[mfernest]

Summary

• Adds six new CDC cookbooks covering the full set of CDC input connectors
• Each cookbook follows a consistent pattern: capture, filter, route to Redpanda, route to S3, route by event type, replication/streaming mode, troubleshoot
• Includes 5 YAML example files per cookbook (30 total) and nav.adoc entries
• Cookbooks added: mysql_cdc, postgres_cdc, mongodb_cdc, microsoft_sql_server_cdc, tigerbeetle_cdc, gcp_spanner_cdc

Resolves DOC-1771.

Test plan

• Local build passes (no errors for new pages)
• All six cookbook pages appear in the nav under Cookbooks
• All included YAML example files resolve correctly
• Netlify deploy preview renders correctly
• CodeRabbit review addressed

🤖 Generated with Claude Code

Page previews

• mysql_cdc cookbook
• postgres_cdc cookbook
• mongodb_cdc cookbook
• microsoft_sql_server_cdc cookbook
• tigerbeetle_cdc cookbook
• gcp_spanner_cdc cookbook