docs: fix Roslyn global tool PATH guidance

[DmitriiFedotovS]

Summary

• document the Roslyn language server's .NET 10 requirement
• explain how to add the global .NET tools directory to PATH
• replace the invalid server version check with command resolution
• clarify that Anycode supplies the required stdio arguments

Verification

• confirmed the installed global tool resolves from $HOME/.dotnet/tools
• ran git diff --check