build: Add checks for Makefile and docs (openedx-unsupported#1047)

Also remove duplicate target from `.PHONY` list.

.github/workflows/quality.yml

@@ -0,0 +1,39 @@
+# Assorted quality checks for PRs.
+
+name: Quality checks
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+    branches:
+      - '**'
+
+jobs:
+
+  run_ci:
+    runs-on: ubuntu-20.04
+    env:
+      DEVSTACK_WORKSPACE: /tmp
+      SHALLOW_CLONE: 1
+    strategy:
+      matrix:
+        python-version:
+          - '3.8'
+      fail-fast: false
+
+    steps:
+      - uses: actions/checkout@v2
+      - name: setup python
+        uses: actions/setup-python@v2
+        with:
+          python-version: ${{ matrix.python-version }}
+
+      - name: Test Makefile
+        run: make selfcheck
+
+      - name: Install Python dependencies
+        run:  make requirements
+
+      - name: Test that docs build without errors
+        run:  make docs

Makefile

@@ -59,7 +59,7 @@
         dev.up.with-watchers dev.validate docs e2e-tests e2e-tests.with-shell \
         help requirements impl-dev.clone.https impl-dev.clone.ssh impl-dev.provision \
         impl-dev.pull impl-dev.pull.without-deps impl-dev.up impl-dev.up.attach \
-        impl-dev.up.without-deps selfcheck upgrade upgrade \
+        impl-dev.up.without-deps selfcheck upgrade \
         validate-lms-volume vnc-passwords
 
 # Load up options (configurable through options.local.mk).