GHA: trim markdown headers before proselinting

Allow interface.md to use more exclamation marks.

Closes #14645

.github/scripts/trimmarkdownheader.pl

@@ -0,0 +1,41 @@
+#!/usr/bin/env perl
+# Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+#
+# SPDX-License-Identifier: curl
+#
+# Given: a libcurl curldown man page
+# Outputs: the same file, minus the header
+#
+
+my $f = $ARGV[0];
+
+open(F, "<$f") or die;
+
+my @out;
+my $line = 0;
+my $hideheader = 0;
+
+while(<F>) {
+    if($hideheader) {
+        if(/^---/) {
+            # end if hiding
+            $hideheader = 0;
+        }
+        push @out, "\n"; # replace with blank
+        next;
+    }
+    elsif(!$line++ && /^---/) {
+        # starts with a header, strip off the header
+        $hideheader = 1;
+        push @out, "\n"; # replace with blank
+        next;
+    }
+    push @out, $_;
+}
+close(F);
+
+open(O, ">$f") or die;
+for my $l (@out) {
+    print O $l;
+}
+close(O);