Add an example implementation of vkGetValue

[JoseEmilio-ARM]

In the "pNext and sType" chapter, so that it is clear how it uses pNext and sType to set the value for b as well as for a.

For example (not sure if this is correct):

{
    a.value = 0;
    if (a.pNext != nullptr) {
        auto b = reinterpret_cast<VkB *>(a.pNext);
        if (b->sType == VK_STRUCTURE_TYPE_B) {
              b->value = 0;
        }
    }
}

[sfricke-samsung]

I thought about this recently too, it would be nice to add a second part explaining that/giving example. I'm happy to write up something first for a review or you can