reuse: add copyright + license info to individual docs/*.md files

Instead of use 'docs/*.md' in dep5. For clarity and avoiding a wide-
matching wildcard.

+ Remove mention of old files from .reuse/dep5
+ add info to .github/dependabot.yml
+ make scripts/copyright.pl warn on non-matching patterns

Closes #13245

scripts/copyright.pl

@@ -135,7 +135,13 @@ sub dep5 {
     while(<F>) {
         $line++;
         if(/^Files: (.*)/i) {
-            push @files, `git ls-files $1`;
+            my @all = `git ls-files $1`;
+            if(!$all[0]) {
+                print STDERR "$1 matches no files\n";
+            }
+            else {
+                push @files, @all;
+            }
         }
         elsif(/^Copyright: (.*)/i) {
             $copy = $1;