Add migration guide for view transformation deprecation (#20338)

# Objective

- add migration guide for #20313

Author: atlv24

release-content/migration-guides/view-transformations.md

@@ -0,0 +1,28 @@
+---
+title: view_transformations.wgsl deprecated in favor of view.wgsl
+pull_requests: [20313]
+---
+
+All functions in view_transformations.wgsl have been replaced and deprecated.
+
+To migrate, a straight-forward copy-paste inlining of the deprecated function's new body suffices, as they all now call the new api internally.
+
+For example, if you had before:
+
+```wgsl
+#import bevy_pbr::view_transformations
+
+let world_pos = view_transformations::position_view_to_world(view_pos);
+```
+
+Now it would be:
+
+```wgsl
+#import bevy_render::view
+
+let world_pos = view::position_view_to_world(view_pos, view_bindings::view.world_from_view);
+```
+
+This was done to make it possible to pass in custom view bindings, and allow code reuse.
+
+`view_transformations.wgsl` will be deleted in 0.18.