Add migration guide pointers for removed policies, tokenizers, and featurizers.

docs/docs/migration-guide.mdx

@@ -99,7 +99,7 @@ class TrainingDataImporter:
 Due to changes in the model architecture the behavior of `rasa train --dry-run` changed.
 The exit codes now have the following meaning:
 
-* `0 means that the model does not require an expensive retraining. However, the
+* `0` means that the model does not require an expensive retraining. However, the
   responses might still require updating by running `rasa train`
 * `1` means that one or multiple components require to be retrained.
 * `8` means that the `--force` flag was used and hence any cached results are ignored
@@ -126,6 +126,24 @@ confidences. This change can affect the maximum confidence predicted by the
 However, the previous behavior can still be enforced by setting
 `ranking_length=10` and `renormalize_confidences=True`.
 
+#### Removed Policies
+
+Several dialogue policies that were deprecated in Rasa 2.x have been removed in Rasa 3.x.
+If you are trying to migrate a config file with a removed policy,
+consult the migration guides for the individual policies:
+
+- `FallbackPolicy` [migration guide](#manually-migrating-from-the-fallback-policy)
+- `TwoStageFallbackPolicy` [migration guide](#manually-migrating-from-the-two-stage-fallback-policy)
+- `MappingPolicy` [migration guide](#manually-migrating-from-the-mapping-policy)
+- `FormPolicy` [migration guide](#forms)
+- `SklearnPolicy` should be replaced with [TedPolicy](/policies#ted-policy)
+
+#### Removed Tokenizers and Featurizers
+
+The `ConveRTTokenizer`, `LanguageModelTokenizer`, and `HFTransformersNLP` featurizer
+components were deprecated in Rasa 2.x and have been removed in Rasa 3.x. See this
+[migration guide](#deprecations-2) for replacing these components in your pipeline.  
+
 ## Rasa 2.7 to 2.8
 
 :::caution