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.
If it works and allows us to customize the header's line then I'm :+1: on that notion. I was just gonna suggest that you could make `^^^^^^` h2 and `----` h3 just for this document but that makes it inconsistent with the rest of the documentation