Aspnetcore.docs: Update topics that use "Shared Framework"

[Rick-Anderson]

• Add brief definition and link to Nate's blog on first use of the term "shared framework".
• Fix incorrect/confusing/misleading usage of the term.

In each of the following docs --

• metapackage.md
• metapackage-app.md
• troubleshoot.md
• 2.0 to 2.1 migration
• 2.2 to 3.0 migration

Create new issue to change the links when https://github.com/dotnet/docs/issues/5009 is completed and there's a doc to link to.

[tdykstra]

As a stop-gap until the https://github.com/dotnet/docs/issues/5009 article is written I'll update these and link to Nate's blog.

[tdykstra]

There are a few other docs that mention shared framework and could use some clarification or a least links. Here then are all of the docs I think we should hit :

• metapackage.md
• metapackage-app.md
• troubleshoot.md
• 2.0 to 2.1 migration
• 2.2 to 3.0 migration

In each --

• Add brief definition and link to Nate's blog on first use of the term "shared framework"
• Fix incorrect/confusing/misleading usage of the term