fix: add documentation for the sdkVersion property

Author: Jake Champion

documentation/docs/fastly:experimental/sdkVersion.mdx

@@ -0,0 +1,14 @@
+---
+hide_title: false
+hide_table_of_contents: false
+pagination_next: null
+pagination_prev: null
+---
+
+# sdkVersion
+
+The read-only **`sdkVersion`** property is a string whose indicates what JavaScript SDK version is being used.
+
+## Value
+
+A string value.