move doc images to relative URLs

The images in the docs all use absolute URLs.
Based on the docusaurus docs they should be relative links.

The relative links are needed to make sure that the images and other static resources work with versioning. The current setup means that we can not have a newer version of an image or update an image without breaking versioned docs.

This will also require the move of images from docs out of the static/img directory in the repo.