Link to the detached signature of the ISO in the torrent documentation

In #11127 (closed) we decided to keep detached signatures of Torrent files, so we should link them from wiki/src/install/inc/steps/download.inline.html.

Related issues

Related to #11127 (closed)
Related to #11121 (closed)
Has duplicate #11516 (closed)

Original created by @sajolida on 11157 (Redmine)

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