Document August 2026 WPF known issue

[rbhanda]

Summary

• document the August 2026 WPF printing and PDF/XPS known issue for .NET 8, 9, and 10
• provide the temporary runtimeconfig.json AppContext switch workaround
• warn that the workaround disables protections introduced in the August 2026 update

Validation

• markdownlint passes for each newly added section
• markdown link checks pass for all three changed files
• diff whitespace check passes with the existing CRLF format in the .NET 10 file