Update illustration-related content to align with Pajamas
Purpose
Updating illustration-related content to align with gitlab-org/gitlab-services/design.gitlab.com!2636 (merged).
- Add illustration-contribution.md doc to document the addition process. This mirrors what we have in the Pajamas repo for updates to the Pajamas UI Kit.
- Update the project README.md to reference icon and illustration guides. Once icon docs are updated in Pajamas a new icon-contribution.md file could be added here too.
- Update the New illustration.md issue template to ensure the checklists align with the contribution process.
Edited by Jeremy Elder