Documentation updates for new configuration system

## Parent PRD

#1660

## What to build

Update all documentation to reflect the new configuration system. This includes:

- **Migration guide**: step-by-step instructions for upgrading from the old config to the new `graph-explorer.config.json` format, including how to use the `pnpm migrate-config` tool
- **Config file reference**: document the full JSON schema with all fields, defaults, and examples for common deployment scenarios (single connection, multiple connections, IAM auth, TLS)
- **Docker deployment docs**: updated instructions using volume-mounted config file, new env vars (`GE_PORT`, `GE_LOG_LEVEL`, `GE_LOG_FORMAT`, `GE_TLS`, `GE_TLS_CERT`, `GE_TLS_KEY`), and simplified Docker Compose examples
- **Development docs** (`docs/development.md`): updated getting-started instructions reflecting zero-config startup, `graph-explorer.config.example.json`, and the new Vite proxy setup
- **Remove references** to old env vars, `config.json`, `defaultConnection.json`, `setup-ssl.sh`, `NEPTUNE_NOTEBOOK`, and the `/explorer` base path throughout all docs

## Acceptance criteria

- [ ] Migration guide with before/after examples
- [ ] Config file reference documenting all fields and defaults
- [ ] Docker deployment docs updated
- [ ] `docs/development.md` updated for new dev workflow
- [ ] All references to old env vars and config files removed from docs
- [ ] README updated if it references old configuration

## Blocked by

- Blocked by #1661
- Blocked by #1662
- Blocked by #1663
- Blocked by #1664
- Blocked by #1665
- Blocked by #1666
- Blocked by #1667

## User stories addressed

- User story 12

## Related Issues

- Part of #1660