Fix discourse embed

Signed-off-by: David Young <davidy@funkypenguin.co.nz>

Dockerfile

@@ -6,4 +6,5 @@ RUN pip install \
         mkdocs-htmlproofer-plugin \
 	mkdocs-git-revision-date-localized-plugin \
         mkdocs-macros-plugin \
-        mkdocs-git-committers-plugin-2
+        mkdocs-git-committers-plugin-2 \
+        mkdocs-meta-descriptions-plugin