ci: Call docs test via top makefile

Doing it this way will catch any errors with generating doc prereqs (like the cmd/cell references).

.github/workflows/test-docs.yml

@@ -39,4 +39,4 @@ jobs:
       - name: Run tests
         shell: bash
         run: |
-          make -C docs test -j${{ env.procs }}
+          make docs DOC_TARGET=test -j${{ env.procs }}