Update the documentation to show how to use from C/C++

[ghaith]

We should add a section in the book explaining how programs and function blocks need to be have their structs and instances declared to be used in C/C++ and how global variables need to be declared as extern to be used. Ideally we show a small example similar to the discussion linked below.

Originally posted by @ghaith in https://github.com/PLC-lang/rusty/discussions/1014#discussioncomment-7554335