Document Debian testing live image for debugging

Debian testing live images are now built weekly:

This addresses one problem we’ve been wanting to solve for years
(#6349 (closed)): very often we would like to ask bug reporters “can you
reproduce this on current Debian testing/unstable?”. Now we can!

I believe that the unofficial GNOME image that includes non-free
firmware is the one we want to point users to in most cases:
http://get.debian.org/cdimage/unofficial/non-free/cd-including-firmware/weekly-live-builds/amd64/iso-hybrid/debian-live-testing-amd64-gnome+nonfree.iso
(signed hashes available in the parent directory).

So:

dear help desk & foundations team: please add these links to some
place where you can find them when needed, and please start
pointing users to them when relevant.

dear doc writers: please consider whether we should link to these
images from some place in our troubleshooting doc.

Feature Branch: doc/12613-debian-live-images

Related issues

Related to #6349 (closed)
Related to #12474 (closed)
Blocks #14758 (closed)

Original created by @sajolida on 12613 (Redmine)

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