Add more specific illustration design guidelines

Problem

https://design.gitlab.com/foundations/illustration is still empty

Proposal

Besides structural differences in empty states (see https://gitlab.com/gitlab-org/design.gitlab.com/issues/115#note_105509332) which will be gradually tackled using the recently introduced empty state component in gitlab-org/gitlab-ui!106 (merged), empty state illustrations still vary in size (live area, bounding box, and more) which leads to more inherited inconsistencies. See gitlab-org/gitlab-ui#177 (moved). Following the need for more specific icon design guidelines in https://gitlab.com/gitlab-org/design.gitlab.com/issues/113, illustration design guidelines could help eliminate inherited inconsistencies.

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Edited by Dimitrie Hoekstra