Lesson Contribution: suggest graphic to support lesson contributions

The following comment was submitted as a lesson contribution outside of GitHub.I won't be able to follow up or provide more details, but the submitter will receive a link to this issue.

re: https://carpentries.github.io/instructor-training/15-carpentries/index.html

> I think it would be valuable to create a visual representation of the Carpentries 'digital ecosystem' as a resource for new instructors. Personally, I noticed I had this strange mental block that kept me from making my first contribution to the curriculum. Upon reflection, I think part of this issue is that I have a hard time holding in my mind the relationship between the carpentries handbook, main website and lesson pages so it was hard to navigate between everything to plan my contribution. A quick visual reference could help new participants keep track of what connects to what.

Start by reviewing the linked lesson at carpentries.github.io/instructor-training/15-carpentries/index.html and the relationships it describes between the handbook, main website, and lesson pages. Define the visual reference's scope and confirm where it should live; done means a clear graphic is available as a resource for new instructors.

Written by the indexing model from the issue text.