Update doc to link to encoding (#5684)

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Co-authored-by: Heath Stewart <[email protected]>

Author: timotheeguerin

website/src/content/docs/docs/language-basics/built-in-types.md

@@ -42,6 +42,16 @@ Built in types are related to each other according to the rules described in [ty
 | `offsetDateTime` | A date and time in a particular time zone, e.g. "April 10th at 3:00am in PST"       |
 | `duration`       | A duration/time period. e.g 5s, 10h                                                 |
 
+:::note
+
+These types do not define any specific serialization format. They represent the concept of time. Each protocol or serialization format should define the default serialization format.
+
+The default encodings for various protocols are defined here:
+
+- [JSON over HTTP](../libraries/http/encoding.md#utcdatetime-and-offsetdatetime)
+- [XML](../libraries/xml/guide.md)
+  :::
+
 ## Other core types
 
 | Type              | Description                                                                                                                                                |