Add built-in paper size catalogs and Avery label support

[rowland]

Summary

• Expand LTML’s built-in page size catalog to cover the common ISO, North American, ANSI, and ARCH families
• Add page-style orientation overrides so samples can request landscape via style.orientation
• Move Avery into a separate extension package with avery:labelsheet and authored avery:label children
• Split the paper-size and Avery demos into dedicated samples and update the syntax docs

Testing

• go test ./ltml
• go test ./avery
• go test ./...