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.
Maybe just take the h2 header lines out of the include fragments and allow ourselves to repeat the headers? That would work.