Add missing doc files and cleanup nits (dotnet/aspnetcore#22931)

* nit: Remove useless `$(HasReferenceAssembly)` settings
  - set in /Directory.Build.targets
    - `true` only in `$(IsAspNetCoreApp)` projects
* nit: Remove useless `$(CompileUsingReferenceAssemblies)` settings
  - no current versioning differences between ref/ and src/ assemblies when targeting default TFM
* Add more `$(GenerateDocumentationFile)` settings
  - increases the number of generated doc files, mostly without problems
- !fixup! correct typo in `DebugProxyHost` doc comments
    - was not generating a doc file before
  - remove previous (ineffective) src/Components/Directory.Build.targets setting
  - nit: remove a duplicate `$(GenerateDocumentationFile)` setting
* nit: Remove useless `$(IsPackable)` settings
  - only analyzers and implementation projects are packable by default
    - main use case for explicit setting is projects shipping only in shared framework
  - conditional setting in src/Mvc/Directory.Build.props just subset logic in /Directory.Build.targets
* nit: Remove useless `$(IsProjectReferenceProvider)` settings
  - only implementation projects are providers by default
* nit: Remove useless `$(IsTestAssetProject)` settings
  - set in src/Mvc/test/WebSites/Directory.Build.props
* !fixup! Looks like `InProcessNewShimWebSite` must compile w/o ref/ assemblies
  - restore `$(CompileUsingReferenceAssemblies)` in this one project

Commit migrated from dotnet/aspnetcore@c4c6e25a06

src/Microsoft.DotNet.Web.Spa.ProjectTemplates.csproj

@@ -5,7 +5,6 @@
     <PackageId>Microsoft.DotNet.Web.Spa.ProjectTemplates.$(AspNetCoreMajorVersion).$(AspNetCoreMinorVersion)</PackageId>
     <Description>Single Page Application templates for ASP.NET Core</Description>
     <PackageTags>$(PackageTags);spa</PackageTags>
-    <IsPackable>true</IsPackable>
     <!-- By default Pack will exclude files that start with '.', we want to include those files -->
     <NoDefaultExcludes>true</NoDefaultExcludes>
   </PropertyGroup>