Add preference to hide announcements/reload-to-update notifications

The notifications that show up on top of documentation in the upper right hand corner appear quite often and are annoying to dismiss. Could there be a preference to either fully silence these or could they be hidden behind a button with a little “new notifications circle” (e.g., like how GitHub shows you that you have unread notifications)? Or perhaps they could just always be hidden behind a clickable notification button and there is no need for a preference.

In my use case, I come to devdocs occasionally but not constantly. So whenever I come, I am trying to read documentation because I can navigate to what I want to read about very quickly but the page is mostly covered by the notifications. I find it annoying to have to keep dismissing them.

Regarding the ~“reload to see new version!” notification, I have no reason to reload immediately because a reload will naturally happen sometime in the future when I close the tab and then open a new tab at an unknown time in the future to find other docs. So I think this notification should also be hideable.

No file, test, or entry point is named. Start by locating the documentation notification UI and reviewing existing notification behavior and settings; agree on one interaction model before implementation. Done should mean the announcements and reload-to-update notification can be unobtrusively hidden or silenced as specified, with coverage for the chosen behavior.

索引モデルが issue の本文から書いたものです。