Add .NET async internals documentation with Mermaid diagrams#64518

Dafydd-E wants to merge 1 commit intodotnet:maindotnet/aspnetcore:mainfrom

Dafydd-E:claude/rebase-aspnetcore-fork-012ATSedvXUcSTP3MiRwgvYBDafydd-E/aspnetcore:claude/rebase-aspnetcore-fork-012ATSedvXUcSTP3MiRwgvYBCopy head branch name to clipboard