Add API diagram

[predictiple]

The diagram itself is not very important. The point of this is really to test the SVG fonts. It contains all 3 fonts so it should be a good test case.

• This diagram was created in my local Excalidraw instance
• then upgraded to be font-compatible with excalidraw.com using my Notebooks.UpdateExcalidrawFontMapping notebook
• then opened and exported to SVG with excalidraw.com so that it contains their font names (which refer to the font definitions that we swap out)
• then using my Notebooks.UpdateSVGfonts notebook I swapped out the font defs, although that will make no difference because Hugo will swap them a 2nd time.