At some point, the Gerrit WebUI ceased displaying SSH host key
details, so go ahead and bake them into our Developer's Guide and
cross-link to them from the appropriate part of the Getting Started
chapter too.
While we're here, add a link from Getting Started to the Developer's
Guide section on the OpenStack ICLA, as users following these
instructions may see errors about that when trying to set up to
contribute to a repository which enforces it.
Change-Id: I9ea5cc9952a0fd4a2bd45fd0ba3e97f8acc7b920