docs: fix code blocks in user-guide/concepts/data-structures

[lukemanley]

Fixes the code blocks on this page so that they are visible: https://docs.pola.rs/user-guide/concepts/data-structures/

Currently, only the output is visible:

[c-peters]

Thanks!

[stinodego]

Thanks for signalling this issue!

The underlying issue here is that the docs/src folder should mimic the structure of the user guide, but it no longer does after the recent updates. We should check and make sure this is aligned again.