docs(UPDATING): add Postgres-targeted maintenance-window queries (sc-105349)

Add a "Sizing the maintenance window on PostgreSQL" sub-section to the
operator runbook. The simple per-table COUNT/duplicate/NULL queries
that were already there are dialect-portable but only count rows;
operators on PostgreSQL with large deployments need to characterize
the migration's runtime cost before scheduling it.

Adds four diagnostic queries:

- Per-table size, row count (from pg_class.reltuples), and which
  migration path each table will take (recreate-rewrite vs direct
  ALTER). Sizes the work concretely.
- Aggregated duplicate-row roll-up: dup_groups + total rows_dropped
  per table. Replaces eight separate per-table queries with one
  consolidated result for audit/dump-before-apply decisions.
- External-FK pre-flight check (the same one the migration runs at
  upgrade time and aborts on). Lets operators surface any blocking
  external reference ahead of the maintenance window. Should be
  empty on a stock install.
- Lock-window estimate for the two full-rewrite tables, using
  pg_relation_size and a conservative 100 MB/s rewrite throughput
  assumption. The other six use direct ALTER and are dominated by
  composite-index build time (seconds for low-millions-of-rows
  tables).

Prompted by reviewer feedback on apache/superset#39859 from a large
deployment asking how to size the maintenance window. The original
pre-flight queries are kept for cross-dialect operators (MySQL,
SQLite) since the new queries use PostgreSQL-specific catalog views.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>