Add C++ example

[aufdenkampe]

As mentioned in #97 (comment):

Unfortunately, @Volk3rJ, @ptomasula, and I had quite a bit of trouble following your previous Babelizer documentation to build https://github.com/csdms/bmi-example-cxx. We're hoping for more success with this meson build approach.

Can you provide a C++ example?

Such an example would help us understand what's happening under-the-hood, especially with how object-oriented C++ objects and functions are exchanged as primitive types among other languages with BMI.

[aufdenkampe]

This example works for MacOS but not on Windows, as mentioned with #105 (comment), but @Volk3rJ had issues on Linux, as mentioned in #107.

Both of us used a conda environment created from the head of the mdpiper/add-cxx-example branch using this file: https://github.com/LimnoTech/babelizer-examples/blob/main/environment.yml.

It would be great to keep up the momentum on fixing these issues with Babelizer and merge these outstanding PRs:

• Simplify README, move content to documentation #100
• Add a C++ example to the documentation #105