Compress all PNG images under doc/

What does this MR do?

Compresses all images. Port of CE https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21329

The pngquant tool was used https://pngquant.org, and particularly, the following command:

/usr/bin/pngquant -f --skip-if-larger --ext .png --speed 1 image.png

• Before: 68000K
• After : 48104K

Author's checklist

• Apply the correct labels and milestone
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards