Parse all `<img>` tags, during doc-gen, and insert the width and height of
the sourceed image, if neither are already specified.
Warnings are reported if the `<img>` tag has no `src` attribute or the image
cannot be loaded.
The work is done in the `addImageDimensions` post-processor, which must be
configured with a `basePath` so that it knows where to find the images.
Closes#15888
Peter Bacon Darwin