Add 4.2 screenshots

[rocodes]

I don't think we have to replace all the screenshots right away, but we should take a pass through the docs and make sure that we at prioritize replace the screenshots that involves the Qubes menu, Devices widget, and other bigger changes.

[eloquence]

Surveying the screenshots in the docs, I'm prioritizing approximately as follows:

• High: Installation screenshots documenting how to copy the submission key from an SVS drive (https://workstation.securedrop.org/en/stable/admin/install.html)
• Medium: System tray screenshots in network troubleshooting guide (https://workstation.securedrop.org/en/stable/admin/troubleshooting_connection.html); maybe others on that page if there are noticeable changes
• Low: Everything else

I didn't spot any screenshots of the main menu, though a bunch of the textual descriptions will need to be updated.

[eloquence]

Collecting first round of screenshots here before I prepare the PR. One caveat for the docs -- when mounting a Tails drive in Thunar, it shows a "permission denied" error for me, presumably because the files are mounted with root-exclusive permissions and I am non-root. I recall seeing this before but I don't know if there's a simple workaround that doesn't involve mucking about with udev rules. Either way though, the files are accessible for the root user.

Device manager with vault VM selected:

Encrypted volume available in Thunar:

Plain Qubes 4.2 desktop with SDW installed:

Updater launched:

Updates in progress:

[eloquence]

Draft PR ready in #219, based off #217 and draft until that's merged.