[DOC] Add code examples
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.
Initial steps in #1800. I filed a follow-up in #1845 to test examples in CI.
Intermediate / advanced example codes are still needed.
Another requested example: using the statistics MR adaptor from C++, similar to the profiler guide for Python: https://docs.rapids.ai/api/rmm/stable/guide/#memory-profiler