Add 'doctest' as a cabal test

We provide a simple wrapper that executes `doctest` on our `src`
directory. This allows us to run all doctests during a regular `cabal
test` run.

package.yaml

@@ -56,9 +56,10 @@ tests:
     build-tools: hspec-discover
     verbatim:
       default-language: GHC2021
-  # doctest:
-  #   main: Doctest.hs
-  #   source-dirs:
-  #     - test/doctest
-  #   dependencies:
-  #     - process
+  doctest:
+    main: Doctest.hs
+    source-dirs:
+      - test/doctest
+    dependencies:
+      - process
+    build-tools: doctest