`memory-layout/guides`: Turn `demos` into `guides`

[teodutu]

There's one guide for the Memory Layout chapter [1], but it's badly formatted and incomplete:

• It should be under guides/, not demos/. config.yaml also needs to be updated to match this change.
• It lacks a README.md explaining how to run it and what to notice.

[1] https://github.com/cs-pub-ro/hardware-software-interface/tree/main/chapters/memory-layout/c-pointers/demos/pointers