Mention the ISO file size on the download page

We should let the user know the size of the .iso before downloading it and advise them to check they have enough free space on their hard drive (also consider the space of the hashsum checker and of the installer they might be asked to download too). Else, they might experience a download failure and get lost.

Feature Branch: web/7417-add-iso-size

Related issues

Related to #9279 (closed)
Related to #9374 (closed)

Original created by @tchou on 7417 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information