Update and improve automated drawbot-skia images

[eliheuer]

These should now include more promotional images for use on social media, and default images should include things like basic character sets and text samples.

The goal is to create automated specimens that can be rebuilt by GitHub actions on major releases. And that the image source files should be checked into the repo for archival reasons. Just like we want reproducible font builds, we should work towards reproducible image and documentation builds.

[eliheuer]

There are some examples of this in a repo here: https://github.com/eliheuer/gf-specimen-images