chapters/data/process-memory/drills/tasks/alloc-size: Move task to the I/O chapter.

[teodutu]

The task asks students to benchmark copying a file with mmap() and read() + write(). This requires using file descriptors as well as calling read() and write() in a loop, which is better suited for the I/O chapter where this approach is explained.

This exercise should be moved to the I/O chapter and replaced with a few exercises focused solely on mmap(): using various access rights and flags, mappings with and without file backings.