v0.5.3 — Docs-only; no behaviour change from 0.5.2

Docs-only; no behaviour change from 0.5.2.

The v0.5.2 tag shipped a README whose digest-pinning example printed a concrete
digest belonging to 0.5.1. Two fixes for that landed after the tag, so the
release was wrong while main was right. A published tag must not be moved.

Adds three metrics caveats that were on internal documentation and not in the
public README: counters are volatile while cache gauges are durable; every
bytes_served_total figure before 0.5.2 is inflated by HEAD requests and must not
be compared across the boundary; and amplification is a derived ratio that needs
its raw counters published beside it.