Add the ability to use the PlantUML `database` shape for systems (including
external systems), containers, and components. The following macros have
been added:
- `SystemDb`
- `SystemDb_Ext`
- `ContainerDb`
- `ComponentDb`
These macros take the same parameters as their regular counterparts.
Adds a new `Enterprise_Boundary` macro based on the system landscape
diagram type from the C4 model (the so-called "Level 0"). Also includes an
example system landscape diagram.