Add Makefile recipe for generating PNGs from Mermaid diagrams

[agrski]

It's easy to forget to generate (up to date) diagrams from Mermaid definitions, especially when there are multiple diagrams per post. This is complicated by the process of adding copyright notices to the generated diagrams using an additional process, as Mermaid does not (yet) support adding captions to all its diagrams formats.

Writing a Makefile recipe would automate this process by capturing the required steps and, if written appropriately, would ensure all diagrams which are out of date relative to their definitions are automatically regenerated when the recipe is run.

In turn, this would support a GitHub Action to detect (or even automatically fix) the issue of stale images for diagrams. This additional functionality would be deferred to another issue, but is mentioned for completeness of context and motivation.