chore: add doc for external types

docs/content/docs/reference/externaltypes.md

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+title = "External Types"
+description = "Using external types in your modules"
+date = 2024-07-12T18:00:00+00:00
+updated = 2024-07-12T18:00:00+00:00
+draft = false
+weight = 110
+sort_by = "weight"
+template = "docs/page.html"
+
+[extra]
+toc = true
+top = false
++++
+
+## Using external types
+
+To use an external type in your FTL module schema, declare a type alias over the external type:
+
+```go
+//ftl:typealias
+type FtlType external.OtherType
+```
+
+The external type is widened to `Any` in the FTL schema, and the corresponding type alias will include metadata 
+for the runtime-specific type mapping:
+
+```
+typealias FtlType Any
+  +typemap go "github.com/external.OtherType"
+```
+
+Users can achieve functionally equivalent behavior to using the external type directly by using the declared 
+alias (`FtlType`) throughout their code. Direct usage of the external type in schema declarations is not supported; 
+instead, the type alias must be used.
+
+FTL will automatically serialize and deserialize the external type to the strong type indicated by the mapping.
+
+## Cross-Runtime Type Mappings
+
+FTL also provides the capability to declare type mappings for other runtimes. For instance, to include a type mapping for Kotlin, you can 
+annotate your type alias declaration as follows:
+
+```go
+//ftl:typealias
+//ftl:typemap kotlin "com.external.other.OtherType"
+type FtlType external.OtherType
+```
+
+In the FTL schema, this will appear as:
+
+```
+typealias FtlType Any
+  +typemap go "github.com/external.OtherType"
+  +typemap kotlin "com.external.other.OtherType"
+```
+
+This allows FTL to decode the type properly in other languages, for seamless 
+interoperability across different runtimes.