[Documentation] Multi-sink logging — observability overview, cross-links and sink decision guide

[pathosDev]

Part of the multi-sink logging effort (meta issue below). Lands after the sinks.

Scope

Each sink issue ships its own documentation section, so this is the finale: the connective
tissue that only makes sense once everything exists.

• observability/overview — logging takes its place next to tracing and metrics, with the
  decision guide: when a single OTLP sink is enough, and when a native sink earns its keep
  (Graylog's gRPC-only OTLP input, Sentry's grouping, Seq's CLEF, local file durability).
• Cross-links: fundamentals/logging ↔ observability/logging/*, and a note on the
  coordinated-shutdown page that log sinks are flushed on terminate() with a bounded
  grace period.
• A short "which sink should I use" table covering the whole matrix, including what the
  framework deliberately does not ship (journald, Windows Event Log — containers scrape
  stdout, which the console sink and JsonLogger already serve).
• README: the logging bullet gains the sink list.
• CHANGELOG: consolidate the wave entries into a coherent Unreleased section, with the
  Security note for MDC sanitization called out.
• Meta issue: close the checklist, move the deferred items into their own follow-up issues
  if they still look worthwhile after the implementation.

Acceptance criteria

• every page exists EN + DE 1:1, code samples identical, prose translated
• scaffold (docs/scripts/scaffold.mjs) and sidebar (docs/astro.config.mjs) in sync
  with every page added across the effort
• in-page links checked (the docs build validates MDX and the sidebar, not links)
• full gate run: bun run typecheck, bun test, bun run test:coverage:gate,
  bun run smoke, plus a docs build
• CHANGELOG and README final pass

Part of #1150.