Add CI job to ensure that the documentation builds without warnings

This will help with issues that are being flagged as warnings by Sphinx.

.github/workflows/ci.yml

@@ -12,6 +12,16 @@ on:
     - cron: 0 0 * * MON # Run every Monday at 00:00 UTC
 
 jobs:
+  docs:
+    name: docs
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-python@v2
+      - run: pip install nox
+      - run: nox -s docs
+
   determine-changes:
     runs-on: ubuntu-latest
     outputs: