[Snapshots] PK snapshot: Docs

[kvch]

What to build

Add user-facing documentation for the pk strategy: selecting strategy: pk; per-table strategy_overrides and primary_keys; the per-batch eventual consistency trade-off and the snapshot-only caveat; and the recommendation to run pgstream check before a pk snapshot. Update the configuration template and the snapshot/configuration guides.

Requirements

• strategy / strategy_overrides / primary_keys documented with examples
• Consistency trade-off and snapshot-only caveat documented
• "Run pgstream check first" workflow documented

Depends on

• [Snapshots] PK snapshot: Walking skeleton: keyset snapshot + batch sizing #928 — Walking skeleton: keyset snapshot + batch sizing
• [Snapshots] PK snapshot: Per-table primary_keys override #931 — Per-table primary_keys override
• [Snapshots] PK snapshot: strategy_overrides + mixed-schema export predicate #932 — strategy_overrides + mixed-schema export predicate