Cross-reference config file failures with the appropriate docs

docs/err/invalid-config-key.rst

 ``dds`` uses a very simple key-value-pair plaintext configuration file format.
 Receiving this error indicates that one of the keys in a configuration file is
 invalid.
+
+..  .
+  TODO: Create reference documentation for packages and libs, and include
+  those links in the seealso below.
+
+.. seealso::
+
+    - For defining a toolchain file, see :doc:`/guide/toolchains` and
+      :ref:`toolchains.opt-ref`
+    - For package and library configuration, see the :doc:`/guide/packages`
+      page.