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".

Author: Bart Koelman

Directory.Build.props

@@ -4,13 +4,13 @@
     <AspNetCoreVersion>3.1.*</AspNetCoreVersion>
     <EFCoreVersion>3.1.*</EFCoreVersion>
     <NpgsqlPostgreSQLVersion>3.1.*</NpgsqlPostgreSQLVersion>
-    <CodeAnalysisRuleSet>$(SolutionDir)CodingGuidelines.ruleset</CodeAnalysisRuleSet>
+    <CodeAnalysisRuleSet>$(MSBuildThisFileDirectory)CodingGuidelines.ruleset</CodeAnalysisRuleSet>
   </PropertyGroup>
 
   <ItemGroup>
     <PackageReference Include="JetBrains.Annotations" Version="2020.3.0" PrivateAssets="All" />
     <PackageReference Include="CSharpGuidelinesAnalyzer" Version="3.6.0" PrivateAssets="All" />
-    <AdditionalFiles Include="$(SolutionDir)CSharpGuidelinesAnalyzer.config" Visible="False" />
+    <AdditionalFiles Include="$(MSBuildThisFileDirectory)CSharpGuidelinesAnalyzer.config" Visible="False" />
   </ItemGroup>
 
   <PropertyGroup Condition="'$(Configuration)'=='Release'">