docs/website: fixing query.yahooapis.com shutdown

This patch rework the use of query.yahooapis.com to do the conversion
from xml to json required by our script and moves to the use of a js
library. Datas are therefore now converted in json format on the client.

Unfortunately, cause of the CORS restriction on nabble and
buildroot.org, we cannot retrieve directly the xml data from these
servers and we need a CORS proxy to do that.

Signed-off-by: Angelo Compagnucci <angelo@amarulasolutions.com>
[Peter: use sha256 integrity]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>