Add chapter around subgroup size control

Either in https://github.com/KhronosGroup/Vulkan-Guide/blob/main/chapters/subgroups.adoc or as a sub-chapter there should be a guide entry around `VK_EXT_subgroup_size_control` (the idea of it)

GPUs support various subgroup sizes and there is sooooo much subtle nuances to it all that it is such a foot gun and should be well documented (with diagrams)

Start with chapters/subgroups.adoc and review its existing subgroup guidance before outlining an entry for VK_EXT_subgroup_size_control. Done means the guide explains the relevant subgroup-size considerations and includes diagrams that make the subtle behavior understandable.

Written by the indexing model from the issue text.