Adds upgrade-v1.2.mdoc and its nav entry, following the structure of the v1.1 guide.

Covers what changed since v1.1.0:

• Migration 000010 — renames deliveries_pkey / deliveries_default_pkey to attempts_*. The server refuses to start with a pending migration, so this is a required step for PostgreSQL log storage.
• RETRY_POLL_BACKOFF_MS redefined (perf(retrymq): cut idle Redis polling to one command per interval #1026) — fixed poll interval defaulting to 100 becomes a maximum idle sleep defaulting to 0 (auto). Only affects deployments that set it explicitly.
• Invalid signature templates now fail at startup (fix(webhook): validate signature templates at startup; build formatters once #1025) — flagged as something to verify in a non-production environment first, since a template that is broken today will stop the server from starting.
• Custom DLQ names (Support custom dead-letter-queue names for self-provisioned SQS/RabbitMQ queues #1033), including what changes for MQS_AUTO_PROVISION=false deployments.
• Per-signal OpenTelemetry resolution (fix(otel): make per-signal exporter and protocol configurable via env #1019), destination-limit 400 (fix: return 400 when max destinations per tenant is reached #1010), CLI error output (fix(cli): print the error before exiting non-zero #1034), delivery connection pooling (fix: share one HTTP client per provider and size its connection pool #1021), Go 1.26.5 and CVE remediation (chore(deps): Go 1.26.5 and dependency refresh for CVE remediation #1030), debian13 base image (chore: upgrade docker base image to debian13 #1036).

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