docs: adding start of migration guide for v3.0.0

MIGRATION.md

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+# Migration
+
+When migrating to a new major version of this SDK, please consult the notes here for breaking changes, new features, and related fixes.
+
+## v2.x.x -> v3.0.0
+
+There are a few breaking changes with v3.0.0 of the Webflow JS SDK
+
+### Breaking
+
+#### CMS APIs
+
+- CHANGED: Deleting a Collection is now achieved through `client.collections.delete()`, which was previously an API used to delete a Collection field.
+    - We've removed `client.collections.deleteCollection()`
+    - To delete a Collection field, use `client.collections.fields.delete()`
+- CHANGED: Creating bulk CMS Items in multiple locales is now achieved through `client.collections.createItems()`
+    - We've removed `client.collections.createItemForMultipleLocales()`
+
+#### Pages APIs
+
+- CHANGED: The `locale` query parameter is now `localeId`
+
+### New Features
+
+#### CMS APIs
+
+- We've added Bulk API support. See [docs](https://developers.webflow.com/data/reference/cms/collection-items/bulk-items/create-items) for more on usage.
+    - `createItems()`
+    - `updateItems()`
+    - `updateItemsLive()`
+    - `deleteItems()`
+    - `deleteItemsLive()`
+
+### Fixes
+
+- Some APIs that created resources (i.e. `collections.items.createItem()`) had types that required an `id`, which is not possible to provide. This is no longer required.
+- `fieldData` type in CMS Items now allows arbitrary extra key/value pairs