Move github banners to assets directory

[bcherry]

Unfortunately #116 didn't work both due to a leading slash in the img tag (which means relative to the host not the current path) and due to the .github/ folder not being exposed from S3/cloudfront. Instead of making changes in readme manager, I changed the doc gen script to also rewrite the image url and move the images into assets where they belong.

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[kath0la]

Thanks, Ben!!