Write recommendation about storing SVG and PNG images

[e-backmark-ericsson]

Description

Update https://github.com/eiffel-community/community/blob/master/howtos/how-to-handle-graphical-images.md to include a statement on the recommendation to store both an SVG and a PNG image representation for each image. We don't enforce updating all existing images at this point.

Also, add recommendation / best practices on how to use InkScape. Mattias has input to this.

Motivation

It makes our image handling conformant

Exemplification

N/A

Benefits

It makes our image handling conformant

Possible Drawbacks

None