Fix cascading polders image in docs

[visr]

@Jingru923 I suggest this as an alternative to #1612. We try to keep this repository as small as possible and therefore prefer to not add images to it. So far we've uploaded images into #1 and used those. Recently GitHub changed something:

Uploading images here won't work anymore for displaying them in our docs, due to GitHub changes, see https://github.blog/changelog/2023-05-09-more-secure-private-attachments/

So using those directly in Markdown no longer seems to work (for newly uploaded images). But @evetion noticed that putting them in an img HTML tag works fine.

[Jingru923]

Looks good.