Updates Based On Feedback From Mailing List

* Remove the QR Code error corrections subsection.

* Remove the non-example Payment Protocol text from developer examples.

* Update reference links and autocrossrefs to reflect above deletions

* Fix CSS padding problems reported by @saivann

* Remove all HTML comments from autocrossref text to allow easy checking
  for broken link definitions: find _site -name '*.html' | xargs grep '\]\['

* Add PNG versions of guide and example SVG icons.  (optipng run)