Add screenshot to locate Disks menu in top navigation bar

During the user testing of VeraCrypt it was really hard for people to locate the Disks menu in the top navigation bar. Probably because people are really not used to it in Linux (it’s more a paradigm from macOS) and there’s usually nothing super helpful or critical in there.

Note for later: we shouldn’t put stuff in there ourselves if we want people to find them.

Note for now: we should add a screenshot in our doc every time we ask people to find it.

• Add a screenshot in the VeraCrypt documentation.
• Add a note about this in the documentation style guide.

Feature Branch: f395fb078bfe3e8c4362368449625fa27214f0d4

Parent Task: #14480 (closed)

Original created by @sajolida on 15668 (Redmine)