General review and updates

Merged Allan Day requested to merge general-review into main

This MR is the result of a general review of most of the content in the HIG. It does 3 things:

  • makes a bunch of small changes to bring the HIG in line with libadwaita 1.0
  • rewrites the navigation page, which I didn't like very much; makes some changes to the windows and browsing pages to go with it
  • a large collection of small style and structure improvements

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