[ci] Use correct ImageMagick command

[ci] Convert PPM image into PNG

[fix] Remove reference to `System.Drawing` assembly

[doc] Use automatically generated preview image

[ci] Generate bigger images

[ci] Add contents write permission

[ci] Change the working directory for deployment

[ci] Checkout branch in a different directory

[ci] Use `actions/checkout` instead of commands

[ci] Ditch Gist and use separate branch

[ci] Remove `gist_file_name` option

[ci] Shrink the image size

[ci] Deploy to a GitHub Gist when new code is available

[doc] Replace the output image with bigger one

[feat] Implement milestone 7 (antialiasing)