[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.

[bdice]

Initial steps in #1800. I filed a follow-up in #1845 to test examples in CI.

Intermediate / advanced example codes are still needed.

[bdice]

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