I have installed Microsoft SQL Server 2017. After installation has completed, I found a folder entitled Visual Studio 2015 in the Document folder. Is is possible to use this Visual Studio? Or this version of visual studio installed because of visual c++? Is it possible to use it with Visual Studio Shell?
SQL Server does not install Visual Studio, anyway you would have found it under C:\Program Files. The files you mentioned are under the Documents folder, so they can't be a working installation of Visual Studio
If you have installed SQL Server Management Studio on the same server, then SSMS probably uses that path to store its settings (latest versions of SSMS are based on the Visual Studio code base).
If this is your scenario you should find a file named NewSettings.vssettings in folder:
...Documents\Visual Studio 2015\Settings\SQL Server Management Studio