update http links

- prefer https

- metadataworkinggroup.org/com seem to be offline; the web archive link
  was obtained from exiftool: https://exiftool.org/TagNames/MWG.html

- fix kramdown link, rubyforge has been gone a long time

- fix png/zlib links

Bug: webp:544
Bug: b/202302177
Change-Id: Id69de4553e7baf00393f12a2c1acb262443a1a93

man/cwebp.1

@@ -1,5 +1,5 @@
 .\"                                      Hey, EMACS: -*- nroff -*-
-.TH CWEBP 1 "November 19, 2020"
+.TH CWEBP 1 "November 17, 2021"
 .SH NAME
 cwebp \- compress an image file to a WebP file
 .SH SYNOPSIS
@@ -222,7 +222,7 @@ Compute and report average PSNR (Peak\-Signal\-To\-Noise ratio).
 .TP
 .B \-print_ssim
 Compute and report average SSIM (structural similarity
-metric, see http://en.wikipedia.org/wiki/SSIM for additional details).
+metric, see https://en.wikipedia.org/wiki/SSIM for additional details).
 .TP
 .B \-print_lsim
 Compute and report local similarity metric (sum of lowest error amongst the
@@ -298,7 +298,7 @@ Please report all bugs to the issue tracker:
 https://bugs.chromium.org/p/webp
 .br
 Patches welcome! See this page to get started:
-http://www.webmproject.org/code/contribute/submitting\-patches/
+https://www.webmproject.org/code/contribute/submitting\-patches/
 
 .SH EXAMPLES
 cwebp \-q 50 -lossless picture.png \-o picture_lossless.webp
@@ -322,5 +322,5 @@ for the Debian project (and may be used by others).
 .BR dwebp (1),
 .BR gif2webp (1)
 .br
-Please refer to http://developers.google.com/speed/webp/ for additional
+Please refer to https://developers.google.com/speed/webp/ for additional
 information.