Docs: Updating Zui

[philrz]

Once the changes from #2866 have been exhaustively tested, we need to update the docs to describe how app updates work.

Things to make sure are covered (not a complete list):

• The default behavior of auto-update (as it has been in the past)
• How the different Settings options control/disable auto-update
• The manual Check for Updates... option in the pull-down menu
• How Linux differs from macOS/Windows

Once this exists, we should link to it from the Installation page.

[philrz]

These docs are now https://zui.brimdata.io/docs/Installation#updates with additional mention higher up on that page.