Add to repo-checklist.md to update github.io and community images page

[e-backmark-ericsson]

Description

On https://eiffel-community.github.io/community.html we have a picture showing all projects in the Eiffel eco-system. When a new project is created (or removed) we should update that picture. To not forget it we should add to repo-checklist.md in this repo that is should be done

Also, a better description on how to handle the new repo graphics should be documented - the xcf file should be updated in the common repo to also include the layer(s) for the new repo color scheme.

Also, add the new png image to https://github.com/eiffel-community/community/blob/master/howtos/how-to-handle-graphical-images.md

Motivation

To keep the eco-system picture and list of community pictures up-to-date

Exemplification

New projects get visualized as soon as they are created

Benefits

New projects get visualized as soon as they are created

Possible Drawbacks

None