Make it easier to consume self-built shared framework bits

[ViktorHofer]

Related: https://github.com/dotnet/core-setup/issues/4809 & dotnet/arcade#2704

The current doc (https://github.com/dotnet/corefx/blob/master/Documentation/project-docs/dogfooding.md) is hacky and works around the fact that we don't have an easy way to consume self-built shared framework bits. I proposed in an offline thread to generate shared framework installers similar to what aspnetcore is already doing. Ideally this would be easier if we can access some of core-setup's logic or by publishing the shared framework generation logic into an own package (see related issues).

cc @SteveCarrollMSFT @geoffkizer @ericstj @danmosemsft

[krwq]

@ViktorHofer on linux this should ideally be as simple as sudo make install to match what other open source projects are doing (or ./install.sh)

[ericstj]

@krwq build and test of a self-built shared framework should not require installing that on the machine. Supporting direct installation from the build is a different issue (but also a good one).

[ViktorHofer]

Triage: we should update the doc for 5.0 infra changes.

[ViktorHofer]

This is documentation only, so I'm moving this into 6.0.