Create documentation page describing the Orthanc/PostgreSQL interface and the conventions

• Write a blog post describing how the Sonador database interface works with the Orthanc PostgreSQL plugin.
  • Explain why additional database tables might be desirable.
  • Introduce SQLAlchemy, declarative_base, and automap_base.
  • Explain how declarative_base and automap_base can be used together to work with the Orthanc managed tables and to introduce plugin managed tables.
  • Show the system in action. Use-case: Sonador resource cache. Speed up query of resources through the use of a PostgreSQL BSON field.