Clarify the difference between a Landing Page address and securedrop.org's .onion address within the Source Guide

[nathandyer]

Because we offer a version of securedrop.org as an onion service, sources see that address (hash) in the URL bar within Tor Browser. When they open the directory and view a listing, they see a different address for a given organization's Landing Page. It's possible that they might mistakenly assume the directory entry they are viewing is the Landing Page, and note that the two addresses don't match, and believe there is a security issue.

We link to the Source Guide in our documentation from directory entries on securedrop.org, so we should probably add an explanation for that within the documentation to help avoid this confusion.