Fix image paths in storage.md documentation

[rgbkrk]

Updated image paths in the storage documentation to be relative to the root.

Annoyingly, when the site renders these images are assumed to be at, for example, /docs/reference/under-the-hood/storage/indexeddb-screenshot-snapshot.png even though storage is not a directory (it's a markdown file). Instead we need to "drop down" a directory to load these files properly in the built version of the site. This isn't great since raw markdown usage is wrong, but then the website is right.

Alternative to this is making the paths to images to use the /docs/reference/under-the-hood/storage/indexeddb-screenshot-snapshot.png path instead of a relative path.