Add diagrams to docs

[sgreenbury]

As discussed in #108, add diagrams for:

• country pipeline (e.g. like the DAG for NI)
• metadata structure (e.g. like the diagram here)

[penelopeysm]

Here's an svg (which you can modify with something like Inkscape if desired 🙂) and the associated png:

edited to change filename_stem -> filepath_stem

svg

png