Convert the RAPI document to restructured text

This patch changes the RAPI document, and the RAPI resources
autogenerated-documentation to restructured text. This meant changing
the autogen tool.

The new fragment can be included via RST directives, and doesn't need
passing through replace-sed-vars. This was also the last sgml document
in doc/, so we remove old makefile rules about it.

Reviewed-by: imsnah