As in this answer, it is possible to use the linked file feature in Visual Studio 2017 to link an entire directory, provided you are using the new .csproj project format and you have the .NET Core 2.0 SDK installed in addition to Visual Studio 2017 15.3+.
<EmbeddedResource Include="..\..\..\docs\**\*.md" LinkBase="docs" />
This will link a docs subfolder using the relative path of your project to a folder within the project named docs (the LinkBase). Every time you add, remove, or change a file in the docs folder or nested folder, the changes will be recognized by Visual Studio (it may prompt you to update the file in Visual Studio if it is already open).

Note that you can use this feature on other MSBuild Project Items besides EmbeddedResource as well, such as Content, Compile, or None.
Projects
Technically, Visual Studio solution files are projects. You can see this by opening the .sln file in a text editor:
Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "build", "build", "{4016BDAB-6C33-4D1E-9439-57B416EA45D5}"
ProjectSection(SolutionItems) = preProject
build.bat = build.bat
build\build.ps1 = build\build.ps1
src\CommonAssemblyInfo.cs = src\CommonAssemblyInfo.cs
TestTargetFramework.proj = TestTargetFramework.proj
Version.proj = Version.proj
EndProjectSection
EndProject
Unfortunately, the .sln format doesn't provide many options. As you can see in Hack the Project and Solution Files, the ProjectSection simply lists the dependencies of the project, and optionally has the ability to specify a "real" relative file path. But that is pretty much it. And according to this similar question, it is not possible to link a directory to another directory in a .sln file.
The only difference between the above Solution Items "Project" and a .csproj project is that the latter shifts control of where the files are added to the .csproj (MSBuild) file.
Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "MyProject",
"src\MyProject\MyProject.csproj", "{3A0AA37E-2B7B-4416-B528-DA4E0E6A6706}"
EndProject
However, as the this answer points out, another workaround is to add a Web Site Project, which will add a project with no .csproj file that keeps track of files on disk "in real time", similar to how the new .csproj format works in Visual Studio 2017.