Update images

The images for next, previous, home pages, etc. are updated to a more up-to-date style. These new images are the same as in the installation guide.

This merge-request is based on the same merge-request done on installation-guide repository (installer-team/installation-guide!11 (merged)).

You can see the differences with both documentations:

• https://www.debian.org/releases/testing/amd64/pr01.en.html (installation guide, new images)
• https://www.debian.org/releases/testing/amd64/release-notes/ch-about.en.html (images needs update)