khepri.app: Add the doc property

Currently, publishing the library documentation to Hex.pm shows the
following warning:

    No valid hex docs configuration found. Docs will will not be generated

I'm not sure what to do about this. According to erlef/rebar3_hex#250,
adding this property fixes or at least works around the message. We'll
how it goes with the next release.

src/khepri.app.src

@@ -19,5 +19,6 @@
 
   {licenses, ["Apache 2.0"]},
   {links, [{"GitHub", "https://github.com/rabbitmq/khepri"}]},
-  {build_tools, ["rebar3", "mix"]}
+  {build_tools, ["rebar3", "mix"]},
+  {doc, "doc"}
  ]}.