Exclude StackOverflow from link checks

StackOverflow sometimes rejects all link check requests from GitHub
Actions, probably due to rate limiting.

docs/documenteer.toml

@@ -40,5 +40,7 @@ sqlalchemy = "https://docs.sqlalchemy.org/en/latest/"
 
 [sphinx.linkcheck]
 ignore = [
-    '^https://github\.com/lsst-sqre/safir/issues/new'
+    # StackOverflow sometimes rejects all link checks from GitHub Actions.
+    '^https://stackoverflow.com/questions/',
+    '^https://github\.com/lsst-sqre/safir/issues/new',
 ]