Document how to specify the path to an image file in a lesson

[tobyhodges]

After https://github.com/carpentries/lesson-example/issues/230 has been closed, add some brief instructions for writing internal links/specifying path to image file. Link to the documentation in the Lesson Example for more details.

This should be added to the Technological Introductions chapter, and linked to from the appendix on converting existing lessons in other formats.