Add solution file and project to make it easier to load docs in Visual Studio (#3496)

Author: jeffkl

docs.sln

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+
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio Version 18
+VisualStudioVersion = 18.3.11122.13
+MinimumVisualStudioVersion = 10.0.40219.1
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "docs", "docs\docs.csproj", "{718966AB-D840-C99F-BC8C-757012BE68AA}"
+EndProject
+Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Solution Items", "Solution Items", "{C2962E4E-20FC-4C00-858E-D0C62611CBFB}"
+	ProjectSection(SolutionItems) = preProject
+		.gitignore = .gitignore
+		.openpublishing.publish.config.json = .openpublishing.publish.config.json
+		.openpublishing.redirection.json = .openpublishing.redirection.json
+		CONTRIBUTING.md = CONTRIBUTING.md
+		global.json = global.json
+		LICENSE = LICENSE
+		LICENSE-CODE = LICENSE-CODE
+		README.md = README.md
+		ThirdPartyNotices = ThirdPartyNotices
+	EndProjectSection
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Release|Any CPU = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{718966AB-D840-C99F-BC8C-757012BE68AA}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{718966AB-D840-C99F-BC8C-757012BE68AA}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{718966AB-D840-C99F-BC8C-757012BE68AA}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{718966AB-D840-C99F-BC8C-757012BE68AA}.Release|Any CPU.Build.0 = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+	GlobalSection(ExtensibilityGlobals) = postSolution
+		SolutionGuid = {C4A337AF-69A4-496C-97FF-300C3489A718}
+	EndGlobalSection
+EndGlobal

docs/docs.csproj

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+<Project Sdk="Microsoft.Build.NoTargets">
+  <PropertyGroup>
+    <TargetFramework>net472</TargetFramework>
+  </PropertyGroup>
+</Project>

global.json

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+{
+    "msbuild-sdks": {
+        "Microsoft.Build.NoTargets" : "3.7.134"
+    }
+}