GHA: add a job scanning for "bad words" in markdown

This means words, phrases or things we have decided not to use - words that
are spelled right according to the dictionary but we want to avoid. In the
name of consistency and better documentation.

Closes #12764

.github/scripts/badwords.pl

@@ -0,0 +1,67 @@
+#!/usr/bin/perl
+# Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+#
+# SPDX-License-Identifier: curl
+#
+# bad[:=]correct
+#
+# If separator is '=', the string will be compared case sensitively.
+# If separator is ':', the check is done case insensitively.
+#
+my $w;
+while(<STDIN>) {
+    chomp;
+    if($_ =~ /^#/) {
+        next;
+    }
+    if($_ =~ /^([^:=]*)([:=])(.*)/) {
+        my ($bad, $sep, $better)=($1, $2, $3);
+        push @w, $bad;
+        $alt{$bad} = $better;
+        if($sep eq "=") {
+            $exactcase{$bad} = 1;
+        }
+    }
+}
+
+my $errors;
+
+sub file {
+    my ($f) = @_;
+    my $l = 0;
+    open(F, "<$f");
+    while(<F>) {
+        my $in = $_;
+        $l++;
+        chomp $in;
+        if($in =~ /^    /) {
+            next;
+        }
+        # remove the link part
+        $in =~ s/(\[.*\])\(.*\)/$1/g;
+        # remove backticked texts
+        $in =~ s/\`.*\`//g;
+        foreach my $w (@w) {
+            my $case = $exactcase{$w};
+            if(($in =~ /^(.*)$w/i && !$case) ||
+               ($in =~ /^(.*)$w/ && $case) ) {
+                my $p = $1;
+                my $c = length($p)+1;
+                print STDERR  "$f:$l:$c: error: found bad word \"$w\"\n";
+                printf STDERR " %4d | $in\n", $l;
+                printf STDERR "      | %*s^%s\n", length($p), " ",
+                    "~" x (length($w)-1);
+                printf STDERR " maybe use \"%s\" instead?\n", $alt{$w};
+                $errors++;
+            }
+        }
+    }
+    close(F);
+}
+
+my @files = @ARGV;
+
+foreach my $each (@files) {
+    file($each);
+}
+exit $errors;