Use better (vector graphics) images; remove unused images from repo automatically

[felker]

Following up from issues discovered when authoring and merging #678

• Add a check to CI to check that all images/*.png, .eps, ... that are currently stored in the repo are linked to by an .md file. Related Need better solution for storing large/many binary assets that arent hosted on main ALCF site #533
• Replace .png diagrams with vector graphics .pdf or .eps versions when possible, especially node diagrams (Add node diagrams to "Machine Overview" pages; move Aurora to top of Machines section in sidebar; move Machine Overview to top of each system dropdown before/after Getting Started page #657)

There is no real reason that we should have low-resolution rasterized graphics like the following, when they were created by ANL folks:
https://docs.alcf.anl.gov/aurora/images/aurora_node_dataflow.png

at a minimum, user higher resolution PNGs

aurora_node_dataflow.png PNG 1134x565 1134x565+0+0 8-bit sRGB 64032B 0.000u 0:00.001