C++/Fotran examples on RTD use outdated BMI

[Peter9192]

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.

[Peter9192]

Might also want to change the g++ command to a cmake command as that's what's used now in all bmi examples and also in the heat examples shipped with grpc4bmi.