Add devices inventory to installation docs [docs] [low-priority]

[rocodes]

Description

As per https://github.com/freedomofpress/securedrop/pull/5273#discussion_r428113099:

We could suggest [locating and creating an inventory of all SD hardware] as part of the install process as well (this could be something tracked on the admin persistent volume)

Can be assigned to me as a low-priority docs issue