Update spi yml for new docs path (#145)

Embedded swift documentation is now hosted on swift.org. This commit
updates the spi.yml file to point at the new host so swift package index
will link to the correct site.

Author: rauhul

.spi.yml

@@ -1,5 +1,3 @@
 version: 1
-builder:
-  configs:
-    - documentation_targets:
-        - EmbeddedSwift
+external_links:
+  documentation: "https://docs.swift.org/embedded/documentation/embedded"