Add entry for VK_KHR_fragment_shading_rate

[Tobski]

We recently noticed that there's no guide entry for this extension - however there is a Khronos blog post here: https://www.khronos.org/blog/khronos-vulkan-working-group-releases-shading-rate-extension-to-increase-rendering-performance-and-quality

Would be good if an entry could be added; much of the text in the blog post can probably be used directly?

[Tobski]

As an additional point, it would probably be useful to draw parallels to the D3D12 "Variable Shading Rate" functionality - noting how these are intended to behave the same.

[janharaldfredriksen-arm]

Related - it would be good to explain the differences between this extension and VK_EXT_fragment_density_map. At first glance, they look pretty similar, but the details are quite different.