Adding linkcheck (and check for other build problems)

Author: bsipocz

tox.ini

@@ -43,7 +43,7 @@ commands =
 
     !buildhtml: pytest --nbval-lax --durations=10 content/
 
-    buildhtml: bash -c "jupyter-book build --execute --html -d"
+    buildhtml: bash -c "jupyter-book build --execute --html --strict -d"
 
 pip_pre =
     predeps: true