docs: field-validated single-node upgrade guide from archived MinIO community edition

[yupoet]

Context

The MinIO community edition is archived (2026-04-25) and its binary channels are gone, so operators on the final community release need a documented path to Silo. The compatibility audit covers server behavior changes well, but does not explicitly address the single-node / single-drive (FS mode) case that most small self-hosted deployments run.

What this adds

docs/upgrade/single-node-from-archived-minio.md — a field-validated upgrade guide based on a production run performed on 2026-09-26:

• why standalone is low-risk (on-disk schema retention → zero migration; single nodes unaffected by the removed storage REST call; IAM/password tightening not applicable to root-credential setups)
• the validated procedure: data-volume backup → pin exact tag → stop dependents → swap image keeping the volume → layered verification (startup log, health endpoints, app-level S3 smoke)
• field results: ~5.9k objects, 15 months of data, zero migration steps, zero objects rejected by the new hardening, full application regression passed
• post-upgrade behavior changes to review per client (TLS defaults across 20260903/20260916, If-Match on DeleteObject now enforced, multipart listing cap, MINIO_API_TRUSTED_PROXIES default)
• honest caveat on rollback: image-swap path exists by schema retention, but a reverse downgrade was not exercised in production — the evidence is a staged forward migration across three engine builds (2025-09 → 2025-10 → 2026-09) on the same volume

Notes for reviewers

• One file, docs-only, no code or behavior change.
• Happy to move/reword/split per maintainers' preference — e.g. if this content belongs on the website repo (silo.pgsty.com compatibility section) instead, I can retarget the PR.