New jquery-doc-fetch-and-generate-data command

This new command is intended to replace the update-doc-data Makefile
step *and* make it easier for users to update the jQuery API data
directly from Emacs.

New var:
  + jquery-doc-api-xml-url

New functions:
  + jquery-doc-fetch-and-generate-data
  + jquery-doc-ensure-api-xml