Introduce C4 Model diagrams for CDSP s/w architecture

After discussion in the Data Architecture team meetings (see Sept-Nov
minutes) the C4 Model (www.c4model.com) was adopted for visualising the
Playground s/w architecture.

This commit introduces the top two levels of abstraction:
1. The level 1 System context diagram which shows how the Playground is
related to the surrounding software systems with which it communicates.
2. The level 2 Container diagram which shows the containers (C4 model
term) which make up the Playground.

Integration note:
Source for these diagrams was the working file used in the team discussions
currently hosted in the group google drive as CDSP-C4-Model.drawio.svg.
That file has multiple tabs, each holding a diagram. The tabs were
exported to separate files to make referencing diagrams easier.

Signed-off-by: Stephen Lawrence <stephen.lawrence@renesas.com>