[ci skip] Refactor libgd docs

Changes to simplify managing libgd docs a bit:
- Move copyright statement and licensing info to common redistributable
  bins file. All previous notices left intact and updated with the
  upstream info.
- Remove unsynced and difficult to track README file compared to
  upstream README.md file available at https://github.com/libgd/libgd

Author: petk

README.REDIST.BINS

@@ -11,7 +11,7 @@
 11. main/snprintf.c
 12. main/strlcat
 13. main/strlcpy
-14. libgd see ext/gd/libgd/COPYING
+14. libgd (ext/gd)
 15. ext/phar portions of tar implementations
 16. ext/phar/zip.c portion extracted from libzip
 17. libbcmath (ext/bcmath) see ext/bcmath/libbcmath/LICENSE
@@ -484,6 +484,76 @@ OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
 ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 
+14. libgd (ext/gd)
+
+* Portions copyright 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001,
+  2002, 2003, 2004 by Cold Spring Harbor Laboratory. Funded under
+  Grant P41-RR02188 by the National Institutes of Health.
+
+* Portions copyright 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003,
+  2004 by Boutell.Com, Inc.
+
+* Portions relating to GD2 format copyright 1999, 2000, 2001, 2002,
+  2003, 2004 Philip Warner.
+
+* Portions relating to PNG copyright 1999, 2000, 2001, 2002, 2003,
+  2004 Greg Roelofs.
+
+* Portions relating to gdttf.c copyright 1999, 2000, 2001, 2002,
+  2003, 2004 John Ellson (ellson@graphviz.org).
+
+* Portions relating to gdft.c copyright 2001, 2002, 2003, 2004 John
+  Ellson (ellson@graphviz.org).
+
+* Portions copyright 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007
+  Pierre-Alain Joye (pierre@libgd.org).
+
+* Portions relating to JPEG and to color quantization copyright
+  2000, 2001, 2002, 2003, 2004, Doug Becker and copyright (C) 1994,
+  1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004 Thomas
+  G. Lane. This software is based in part on the work of the
+  Independent JPEG Group. See the file README-JPEG.TXT for more
+  information.
+
+* Portions relating to GIF compression copyright 1989 by Jef
+  Poskanzer and David Rowley, with modifications for thread safety
+  by Thomas Boutell.
+
+* Portions relating to GIF decompression copyright 1990, 1991, 1993
+  by David Koblas, with modifications for thread safety by Thomas
+  Boutell.
+
+* Portions relating to WBMP copyright 2000, 2001, 2002, 2003, 2004
+  Maurice Szmurlo and Johan Van den Brande.
+
+* Portions relating to GIF animations copyright 2004 Jaakko Hyvätti
+  (jaakko.hyvatti@iki.fi)
+
+Permission has been granted to copy, distribute and modify gd in
+any context without fee, including a commercial application,
+provided that this notice is present in user-accessible supporting
+documentation.
+
+This does not affect your ownership of the derived work itself,
+and the intent is to assure proper credit for the authors of gd,
+not to interfere with your productive use of gd. If you have
+questions, ask. "Derived works" includes all programs that utilize
+the library. Credit must be given in user-accessible
+documentation.
+
+This software is provided "AS IS." The copyright holders disclaim
+all warranties, either express or implied, including but not
+limited to implied warranties of merchantability and fitness for a
+particular purpose, with respect to this code and accompanying
+documentation.
+
+Although their code does not appear in the current release, the
+authors wish to thank David Koblas, David Rowley, and Hutchison
+Avenue Software Corporation for their prior contributions.
+
+END OF COPYRIGHT STATEMENT
+
+
 15. ext/phar portions of tar implementations
 
 portions of tar implementations in ext/phar - phar_tar_octal() are based on an