doc: corrected NEWS.xml typo
doc: updated Lua bindings link in README.xml
doc: refactored to be more mainstream

Dropped custom XSL and merged latest gtk-doc.make into
Makefile.am.gtk-doc: there is no more need to have XML 1.0 compiant
pages because the new online website is based on silverstripe-gtkdoc
(http://dev.entidi.com/p/silverstripe-gtkdoc/) that accepts HTML code.

Corrected minor bugs during the process:
* removed wikilinks in @short_description because they are not properly
  handled by gtk-doc;
* removed trailing dots from @short_description for consistency;
* added index of new APIs introduced by 1.0 version;
* added index of deprecated APIs (actually empty);
* merged docs/gnu into docs/adg;
* removed docs/common.