Explain what the white and grey layers are on the tree page

[Rafiot]

What is changing?

We need to explain what the white and grey lines are on the tree page.

Note: they are used to show that a node (on the left) is loading content (on the right). Not having the background makes the tree weirder to read, so it needs to be there, but we need to explain it somehow.

How will this impact users?

Better understanding of the tree.

[stale[bot]]

Close call! This issue has been marked as stale because it has not had any recent activity. It should be closed if no further activity occurs. Add a comment or push a commit to keep this issue stay alive and kicking. Thank you for your contribution; it is appreciated.