docs: add a notice about the portable_pdb descrition being moved to the https://github.com/dotnet/designs repo #9022
Description
Description
Like https://github.com/dotnet/core/blob/main/Documentation/diagnostics/source_link.md page which has a notice that points to the new location at https://github.com/dotnet/designs/blob/main/accepted/2020/diagnostics/source-link.md
The page at https://github.com/dotnet/core/blob/main/Documentation/diagnostics/portable_pdb.md should also have a notice that points to the new location at https://github.com/dotnet/designs/blob/main/accepted/2020/diagnostics/portable-pdb.md