Fixed warning when building documentation
Fixed warning when building documentation

When running docfx, it builds only the JsonApiDotNetCore project, not the entire solution. Because of this, the use of $(SolutionDir) msbuild variable produces the next warning:

  Could not find rule set file "CodingGuidelines.ruleset"

This commit replaces $(SolutionDir) with the magical $(MSBuildThisFileDirectory) msbuild variable, which evaluates to: "the directory where the file exists that references this variable".

Research-Institute/json-api-dotnet-core,Research-Institute/json-api-dotnet-core,json-api-dotnet/JsonApiDotNetCore