Move "generate SSH key" and "make discovery-url" sections inline to docs

I think including these sections inline to each install doc makes the install workflow easier to follow and less error-prone. Using reStructuredText include fragments keeps it DRY. I also reworded the "generate discovery URL" section for economy.

Closes #2510, refs #2473.
Good point, I'll have to think about how to do that without repeating things.