ci: install sphinx into the CI image

.gitlab/ci/docker/fedora31/install_deps.sh

@@ -10,4 +10,10 @@ dnf install --setopt=install_weak_deps=False -y \
     gcc-c++ \
     git-core
 
+# Install documentation tools.
+dnf install --setopt=install_weak_deps=False -y \
+    python3-sphinx \
+    texinfo \
+    qt5-qttools-devel
+
 dnf clean all