The whole purpose of showing a notification on the `sidebarToggle` button, when the sidebar is closed, was to give users *some* kind of indication that the PDF document contains outline/attachments/layers without having to manually open the sidebar to check.
However, in the implementation in PR 7959, I also added notifications for each view-buttons in the sidebar. Looking back at this, I've always questioned the value of the last part, since the view-buttons already have a `disabled`-state which shows if they're available or not. Hence we're actually, in a sense, duplicating notifications for the outline/attachments/layers-buttons without adding (in my opinion) all that much overall value.
All-in-all, I'm thus proposing that we only display the notification on the `sidebarToggle`-button itself, since that should really be sufficient here, which also allows us to simplify the relevant code a fair bit.
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