Reorganize interop articles

[nxtn]

Status

Several problems with interop topics in .NET docs.

• Scattered and repetitive. There are identical/similar contents in .NET Guide and .NET Framework Guide.
• Incomplete. For more information, see Identify archived interop topics to migrate back to docs.microsoft.com #4649.

Rationale

• P/Invoke is still widely adopted.
• CoreCLR has enabled us to embed .NET Core into native code.
• The behaviors apply to both .NET Framework and .NET Core.
• The behaviors apply to both P/Invoke method and calli instruction.
• The behaviors apply to both calling from managed code to native code and the reverse.

Proposal

• Migrate archived interop topics back to docs. Identify archived interop topics to migrate back to docs.microsoft.com #4649
• Integrate and reorganize topics in .NET Guide and .NET Framework Guide. Ensure our cross-platform docs are not Framework-centric and Windows-centric any longer dotnet-api-docs#1199 and Interop Marshalling moved #6629.
• New topics about calling from native code to managed code. .NET Core hosting
• Keep consistent with C++ docs if possible.

[rpetrusha]

@NextTurn, thanks for opening this issue. I also added a link to #6629 to the Proposal section. We'd welcome your contribution in improving the interop documentation, and your proposal looks really good. Before actually migrating archived content, it's a good idea to agree on the topics that are to be migrated.