Change downloads to (iso + checksum + link to gpg key)

In this age of cracking and insecurity it would probably be a good idea to make the download a bundle containing all you need to properly install Trisquel. The archive should contain

* The actual disk image
* Checksums
* Link to the Trisquel GPG key
* Instructions what to do with them

We have some related user submitted instructions here
* https://trisquel.info/en/wiki/burn-trisquel-cd-images (currently linked to from the download page)
* https://trisquel.info/en/wiki/how-create-liveusb (currently linked to from the download page)
* https://trisquel.info/en/wiki/verify-trisquel-download