Delete obsolete documentation

[MichaelSimons]

First pass at deleting documentation that is obsolete. This will help avoid confusion as this documentation pollutes the repo and can be misleading.

[mthalman]

While most of this content is obsolete, I think there are some parts worth keeping such as "Toolset for Building .NET Core" and "N-1 Toolset".

[MichaelSimons]

The "N-1 Toolset" is covered by https://github.com/dotnet/source-build/blob/main/Documentation/bootstrapping-guidelines.md#building-a-servicing-release-of-net. Specifically the n-1 use in servicing is addressed in https://github.com/dotnet/source-build/blob/main/Documentation/bootstrapping-guidelines.md#building-a-servicing-release-of-net and the need to rebootstrap in new major builds is covered in the opening.

I agree the "Toolset for Building.NET Core" does have some content that should be preserved. Thinking about where to put this...

[MichaelSimons]

I preserved the distro packaging guideline exception in a7811f4

[mthalman]

I agree the "Toolset for Building.NET Core" does have some content that should be preserved. Thinking about where to put this...

Is the plan to update this PR to include that or do that in a separate PR?