add mathwork sign to ignore list

Author: cyschneck

conf.py

@@ -384,4 +384,7 @@
 linkcheck_anchors = False
 
 # ignore some links that work, but are disliked by linkcheck
-linkcheck_ignore = [r'https://doi.org/10.1080/104732299303296']
+linkcheck_ignore = [
+    r'https://doi.org/10.1080/104732299303296',
+    r'https://www.mathworks.com/help/matlab/ref/double.sign.html',
+]

ncl/ncl_entries/general_applied_math.ipynb

@@ -912,7 +912,7 @@
    "source": [
     "## sign_matlab\n",
     "\n",
-    "NCL's `sign_matlab` mimics the behavior of the Matlab [sign](https://www.mathworks.com/help/simulink/slref/sign.html) function\n",
+    "NCL's `sign_matlab` mimics the behavior of the Matlab [sign](https://www.mathworks.com/help/matlab/ref/double.sign.html) function\n",
     "\n",
     "```\n",
     "1  if the corresponding element of X is greater than zero\n",