C++/Fotran examples on RTD use outdated BMI

The instructions on adding a BMI server for c++/fortran models uses quite old BMI definitions, which make it confusing.

https://github.com/eWaterCycle/grpc4bmi/blob/main/docs/server/Cpp.rst

For example:
- Latest bmi c++ uses Pascal Case for method names
- Latest bmi c++ uses std:string instead of char*
- Latest bmi fortran uses `bmif_get_conmponent_name` instead of `get_component_name_f`
- Latest bmi c++ heat example uses extensions `.cxx` and `.hxx`. It would be nice to be consistent with the heat example.