examples: cpp: Add further instructions

[RaulTrombin]

No description provided.

[joaoantoniocardoso]

for me, they are still build instructions.

[RaulTrombin]

We can add more instructions here,
such create&use a container that cross-compile it

And follow the same title as Python instruction: https://github.com/bluerobotics/navigator-lib/blob/master/examples/python/README.md

[joaoantoniocardoso]

I'd say that the title of the 'Python instructions' document is not great, and in the future, if you add a "create&use a container that cross-compile it" instruction, you would need to create a different subsection anyway, then change the title when it makes sense.