0.11 to 0.12 migration guide

[IceSentry]

This is still very WIP and only contains the original output of the generate-release tool.

Feel free to submit a PR to my branch with any fixes.

[rparrett]

Notes (will update as I migrate stuff and the guide evolves):

• Transparent2d has a new dynamic_offset field.
  Seems like this should be set to None.
• Transparent2d's batch_range field is now a range. Maybe it should be 1..1 if it was previously None, and you are not doing automatic batching?
• PrepareAssetSet is gone?
  This seems to be covered by "Assets such as materials and meshes should now be created" but maybe mention the set by name.
• I think that a particular system previously in Queue should now be in QueueMeshes but I don't see QueueMeshes mentioned anywhere.
• We should explicitly explain what to do to migrate ComputedVisibility::is_visible. I think that it did not consider view visibility so it is probably equivalent to InheritedVisibility::get?
• Frustum::intersects_obb now takes an Affine3A instead of Mat4.
  GlobalTransform::affine() may help? No mention at all of Affine3A in guide.
  Expand on migration guide for PR 9416 #816
• PreparedBindGroup bindings field's inner type is now a tuple with a mysterious u32 field.
• assets v2: assets.set_untracked is gone. No mention in guide.
  Seems like maybe you just use .insert now?
• assets v2: app.add_asset is gone, but not mentioned explicitly.
  Seems to be init_asset now.
• assets v2: loaders now need an associated Error type. no mention of this.
• assets v2: section on implementing a custom loader should link to new custom loader example
• assets v2: get_group_load_state is gone.
  Use .iter().any() on your collection with get_load_state, I guess.
  Examples seem to use e.g. Assets<Image>::get().is_some() though... is there a reason for that?
• opt-out multi-threading: This is not just for users who depend on individual bevy crates. Users who disable bevy's default features in theirs apps need to add this feature. Also, if you don't, your assets will just silently never load / be stuck in a perpetual LoadState::Loading.
  0.12 Migration: multi-threaded feature is not just for bevy_ecs and bevy_tasks users #830

[cart]

Thanks a ton for putting this together! I think this is in a reasonable place. We can iterate as needed (even post-release if necessary)