Add preloaded disposables images

[ben-grande]

For: QubesOS/qubes-posts#146
For: QubesOS/qubes-issues#1512

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This repo will not be relevant once the migration to the RTD completes, use qubes-doc/attachments instead (check rst/rtd branch).

Reminder that images must be updated, they don't show the threshold feature.

[maiska]

you won't need the images for documentation? just for posts?

[ben-grande]

you won't need the images for documentation? just for posts?

When the feature is documented, it will make sense to reuse the same images from the post. Will that mean that these images needs to be on 2 different repos? Please see recent (yesterday) chat on Matrix regarding documentation. I think it is up to documentation contributors to updating the disposable pages, even if reusing these same images (if they are still up to date).