docs: refresh README for shipped Doctrine + Symfony bundle

[erikfrerejean]

Summary

The README still described the package as core-only, with Doctrine persistence and the Symfony bundle "landing in subsequent releases." Both have already shipped, so the README was materially out of date.

This refreshes it to match reality:

• Status note — replaced the stale "core-only" claim with the bundle's actual scope (framework-free core + Doctrine/Symfony adapters), linking ADR-0009.
• Requirements — now lists the real Symfony 8.0 + Doctrine (DBAL ^4, ORM ^3.6, doctrine-bundle ^3.2) dependencies instead of just psr/clock/psr/log.
• Installation — adds the manual config/bundles.php registration step (no Flex recipe) and links docs/installation.md for the migration and append-only DB grant.
• Design — adds the read side (RecordReader) to the responsibilities table.
• Usage — notes that $recorder is bundle-autowired and points to SubjectLabeller / RecordReader wiring.

The UserRoleChanged code example was left untouched — verified still accurate against the current AbstractAuditEvent and FieldChange.

Notes

• Install steps are deferred to docs/installation.md rather than duplicated, to avoid the two docs drifting (the config/bundles.php snippet does appear in both — small duplication surface).

Test plan

• Docs-only change; claims verified against composer.json, the src/ tree, and docs/installation.md.