Adds the JasperFx.Events.Documents persistence abstractions (#647) — the
document slice JasperFx.Events did not previously cover, so store-agnostic
consumers can read and write documents alongside the event store.

Purely additive: new namespace, no existing public type changed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ri7jHRKhaFNBesQM1B5i1k