Update banner & add support notice. #61
                
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Adds a new banner on all pages, linking to an extended support notice on the home page.
Staged: https://20230111t151412-dot-polymer-library.uc.r.appspot.com/3.0/docs/devguide/feature-overview
Newer version staged: https://20230117t111709-dot-polymer-library.uc.r.appspot.com/3.0/docs/devguide/feature-overview