docs: document multi-instance dev launcher

[kelsos]

Summary

Documents the new multi-instance dev launcher (pnpm dev --instance <name>) in the frontend contribution guide.

The new Running Multiple Instances section in working-with-frontend.md covers:

• Starting an instance — --instance <name>, bare --instance (derives from $INSTANCE_NAME/git branch), and --no-instance.
• Port allocation — slot model, base port 13000, the dev/backend/proxy/colibri layout, the .port-index.json registry, and reserved-port skipping.
• Data directories — <app-data>/rotki-dev/<name>, first-run seeding, and the --no-seed, --include-backups, ROTKI_SEED_SOURCE, ROTKI_DEV_INSTANCES_DIR knobs.
• Managed env block — the sentinel-wrapped block written to .env.development.local and the --accept-managed-env consent flow.
• Lifecycle commands — --list, --clean, --clean-all, --prune, --repair, plus --yes / --older-than, the live-instance guard, and clean Ctrl+C shutdown.

Pairs with the frontend change in rotki/rotki (feat(frontend): multi-instance dev launcher).