C++ devguide should include necessary #includes

[skye]

I'm looking at https://abseil.io/docs/cpp/guides/strings, and it would be very useful if it told me which #include I need for each function. It looks like all of the guides could benefit from this.

[axgillies]

This would require a larger-scale change to the way that we provide samples in all of the Tips. Assigning to dmauro for some additional input.