Add CI test for man page updates (#3729)

Run make -f Makefile.doc in CircleCI to test that man pages are updated
in PRs.

Signed-off-by: Ben Kochie <superq@gmail.com>

fixup_file_mtime.sh

@@ -0,0 +1,15 @@
+#!/usr/bin/env bash
+#
+# Description: Fix up the file mtimes based on the git log.
+
+set -u -o pipefail
+
+if [[ ! -f 'coredns.1.md' ]]; then
+  echo 'ERROR: Must be run from the top of the git repo.'
+  exit 1
+fi
+
+for file in coredns.1.md corefile.5.md plugin/*/README.md; do
+  time=$(git log --pretty=format:%cd -n 1 --date='format:%Y%m%d%H%M.%S' "${file}")
+  touch -m -t "${time}" "${file}"
+done