docs(snapshots): aria2c installation prerequisite missing after switching from wget

The snapshot download commands were recently updated to use aria2c instead of wget. However aria2c is not installed by default on Ubuntu and the page has no prerequisite section mentioning this. A new operator following the guide will hit 'command not found: aria2c' when running the download command.

wget is part of the standard Ubuntu install but aria2c requires apt-get install aria2 first. Adding a short note before the download table would prevent this confusion.