Sort 0.16 migration guide entries by order of importance

[BD103]

The order of the migration guide entries in _guides.toml affect their order of appearance in the rendered page. It would be beneficial to skim through each area in the migration guide and move the most impactful changes to the top of their area's section. (For example, the entity relations guide should be moved to the top of the ECS area section.)

[BD103]

I'm adding this to the 0.16 milestone, though it can be cut or closed if deemed unimportant enough. Do note that this should happen last, as this kind of re-ordering is guaranteed to cause merge conflicts!

[alice-i-cecile]

Yep, I'm on board with this. A rough pass is fine though.