doc/doc-setup.org: Absorb header from Emacs repo

In the Emacs repository, this file's header was updated in include
copyright information in b3d310fa96 (Fixes for doc/misc org source
files, 2021-02-26).  Copy it over.

doc/doc-setup.org

@@ -1,4 +1,21 @@
-# SETUPFILE for manuals
+# SETUPFILE for Org manual
+
+# Copyright (C) 2021 Free Software Foundation, Inc.
+#
+# This file is part of GNU Emacs.
+#
+# GNU Emacs is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# GNU Emacs is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
 
 # XXX: We cannot use TODO keyword as a node starts with "TODO".
 #+todo: REVIEW FIXME | DONE