Clean-up pkgproj leftovers in libs and apply fixes (#56899)

* Clean-up pkgproj leftovers in libs and apply fixes

Remove any pkgproj infrastructure that was only used by src/libraries
(which is the majority). Delete the packageindex *YAY*.
Update the documentation that covered the packageindex and the pkgprojs.

Avoid any incremental builds during packaging by removing
libraries-packages.proj and use src.proj for packaging instead. Make use
of the `GeneratePackageOnBuild` property to build package during the
allconfigurations without requiring a different entry target.

Fix the addition of the DocumentationFile during packaging when
`GeneratePackageOnBuid` is used by hooking onto the
`DocumentationProjectOutputGroup` that NuGet uses and replacing the
generated documentation file with the one that comes via the
intellisense package.
Also introduce a property to choose the generated documentation file
over the one from the intellisense package:
<UseIntellisenseDocumentationFile>false</UseIntellisenseDocumentationFile>

Removing a few leftover PackageDescription properties from the projects'
Directory.Build.props file.

Cleaning up properties in Directory.Build.props & Directory.Build.targets
files.

* Actually run packaging during the allconfigurations build

* Update docs

* make runtime specific pkgs non packable

* io.ports native pkg fixes

docs/coding-guidelines/project-guidelines.md

@@ -151,9 +151,6 @@ All src outputs are under
 
 `bin\$(MSBuildProjectName)\$(TargetFramework)`
 
-## pkg
-In the pkg directory for the library there should be only **one** `.pkgproj` for the primary package for the library. If the library has platform-specific implementations those should be split into platform specific projects in a subfolder for each platform. (see [Package projects](./package-projects.md))
-
 ## tests
 Similar to the src projects tests projects will define a `TargetFrameworks` property so they can list out the set of target frameworks they support.