ci: add job for documentation build and test

This adds another job to the CI runner. This builds and tests the
documentation. The tests are currently reduced to just verifying the
respective man-pages are actually generated. This can be extended on in
the future.

.github/workflows/tests.yml

@@ -30,6 +30,36 @@ jobs:
           cd osbuild
           pylint osbuild runners/* assemblers/* stages/* sources/*
 
+  documentation:
+    name: "📚 Documentation"
+    runs-on: ubuntu-latest
+    container:
+      image: docker.io/library/python:3.7
+    steps:
+      - name: Install Dependencies
+        run: |
+          pip install docutils
+
+      - name: Clone repository
+        uses: actions/checkout@v2
+        with:
+          path: osbuild
+
+      - name: Generate Documentation
+        run: |
+          make \
+            -f osbuild/Makefile \
+            SRCDIR=osbuild \
+            BUILDDIR=build \
+            RST2MAN=rst2man.py \
+            man
+
+      - name: Verify Documentation
+        working-directory: build
+        run: |
+          test -d docs
+          test -f docs/osbuild.1
+
   unit_tests:
     name: "unit"
     runs-on: ubuntu-latest