Decide if want to have a download only page on /install/download

The installation assistant replaces /download and people are supposed to following installation scenarios from the beginning until the end. Still, right now we have https://tails.boum.org/install/download/ and 2471f1a in web/9323-release-ia adds it as a link from the documentation index.

Related issues

Related to #11024 (closed)

Original created by @sajolida on 10938 (Redmine)

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