[DOC] Add code examples

[bdice]

Currently RMM does not provide any self-contained code examples like cudf or cuvs.

New users would benefit from seeing minimal, correct, and educational patterns of C++ usage and CMake build infrastructure.

One specific example we should include is using RMM in a multi-thread, multi-stream application, as that is a common request that our documentation does not serve well today.

The code examples should build and be run in our CI, as we do for cudf and cuvs.