Reworked the download page to link from "add this key to your apt

Reworked the download page to link from "add this key to your apt
configuration" to the /archive-key page, pushing the SecureApt
reference link into that page. This has a streamlining affect on new
users wanting to get started as it links directly to the archive-key
page where there are explicit instructions for how to do the archive
key verification and then the follow-up add to the apt keyring for
proper archive/package verification. The way it was before, you were
given the link to the more general SecureApt debian wiki page, which
was a lot of information, when in most cases people just want to know
what steps they need to know to do the verification/addition. However,
the link to the SecureApt page is valuable to reference, it just makes
more sense to be embedded in the page which explicitly deals with
that.

This also means I added information about how to add the key to the
apt keyring into the archive-key page. In doing so, I removed the
dkg-specific prompt (not because I dont like it, but because it would
have been funny for me to add more commands and have to manually
construct his prompt to maintain consistency on the page). Also, added
some output of commands so people have an idea what to expect.