Print instructions for SSH Deploy Keys

[adrhill]

As discussed in #460 and documented in the Documenter docstring, setting up a new package currently requires manually running

using DocumenterTools
DocumenterTools.genkeys(user="MyUser", repo="MyPackage.jl")

which prints instructions on how to add the DOCUMENTER_KEY SSH key to GitHub or Travis.

This step could be automated for templates using both Documenter and GitHubActions plugins (or Documenter and TravisCI) by printing the output of DocumenterTools' genkeys at the end of the template generation.