add a figure on database as a collection of connected tables

[avallecam]

here (after introducing DBI in the "reading from databases" section) we should add a figure to define a database as a collection of connected tables. This figure could be used as an initial addition and then edit a new one based on it.

(source: https://jcsites.juniata.edu/faculty/rhodes/dbms/relnmodel.htm)

Originally posted by @avallecam in #70 (comment)