Backup/restore: admin guide section for backing up SQLite and PostgreSQL with their own tools #750

A Database Backup and Restore section in docs/manuals/05-admin-guide.md with commands measured on SQLite and PostgreSQL, pointers to it from the upgrade manual, the security model and the deployment guide, and the admin guide's Data Recovery entry as a pointer to the section. Outside the design's PR numbering; design PR 11 rewrites the manuals' backup sections once the store-backed path has shipped.

Part of #624. Implemented in #737.