Added missing exception mechanism property. (#12929)

The `source` property as described in the RFC was missing: https://github.com/getsentry/rfcs/blob/main/text/0079-exception-groups.md#source

develop-docs/sdk/data-model/event-payloads/exception.mdx

@@ -111,6 +111,14 @@ the root exception (the last to be listed in the exception values).
 
 : An optional flag indicating that this exception is part of an exception group type specific to the platform or language.
 
+`source`
+
+: An optional string value describing the source of the exception. The SDK should populate this with the name of the property or attribute of the parent exception that this exception was acquired from. In the case of an array, it should include the zero-based array index as well.
+
+- Python Examples: `"__context__"`, `"__cause__"`, `"exceptions[0]"`, `"exceptions[1]"`
+- .NET Examples: `"InnerException"`, `"InnerExceptions[0]"`, `"InnerExceptions[1]"`
+- JavaScript Examples: `"cause"`, `"errors[0]"`, `"errors[1]"`
+
 `meta`
 
 : Optional information from the operating system or runtime on the exception