Visual Studio doesn't allow you to just make a solution. You are forced to make a project as well. This doesn't lend to flexibility in folder structure.

Edit: There exists a sln template that can now be created with dotnet new sln. The contents of that template are replicated here.

I like to start with a blank solution file. The blank lines are intentional.

Microsoft Visual Studio Solution File, Format Version 12.00
# Visual Studio 15
VisualStudioVersion = 15.0.26124.0
MinimumVisualStudioVersion = 15.0.26124.0
	GlobalSection(SolutionConfigurationPlatforms) = preSolution
		Debug|Any CPU = Debug|Any CPU
		Debug|x64 = Debug|x64
		Debug|x86 = Debug|x86
		Release|Any CPU = Release|Any CPU
		Release|x64 = Release|x64
		Release|x86 = Release|x86
	GlobalSection(SolutionProperties) = preSolution
		HideSolutionNode = FALSE

Just plop that into a new file, slap .sln on the end of whatever you decide to name it, open it in Visual Studio, and you'll have a blank solution from which you can start clean.

The next step is often to create blank project files, and you can learn how to do that in this post.