Make the OPT recomendation clearer

Make the OPT recomendation clearer

AFAIK OTP releases are the recomended way of installing, but

  * People seem unaware of that and use from source installations because they use the guide with the name of their distro
  * People don't know what OTP releases are or what it means

I added a warning on all installation-from-source guides and added the same explanation on the two OTP pages (the miigration to OTP and installing OTP)