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
This is to tidy up the `author-packagse`, which currently duplicates a
lot of the configuration in the main packages. We need to
DRY this up so that we don't fall foul of a change in one being missed in
the other.
