add initial docs

docs/edbob.db.rst

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+
+:mod:`edbob.db`
+===============
+
+.. automodule:: edbob.db
+
+.. attribute:: engines
+   
+   A dictionary of configured :class:`sqlalchemy.Engine` instances available to
+   the application.
+
+.. attribute:: engine
+
+   A reference to the default :class:`sqlalchemy.Engine` instance for the
+   application.
+
+.. attribute:: metadata
+
+   A reference to the global :class:`sqlalchemy.MetaData` instance for the
+   application.
+
+.. class:: Session
+
+   A :class:`sqlalchemy.orm.Session` class for use within the application.
+
+.. autofunction:: get_setting
+
+.. autofunction:: save_setting
+
+.. autofunction:: needs_session