Adjust location of fast SSH lookup content in Geo docs

What does this MR do?

I've noticed that customers on single-node Omnibus Geo installations are frequently missing the steps to switch to using fast SSH lookup. I took a look and in the single-node site docs, this item is under the section's H2 rather than its own H3. I moved it to its own H3 so it's hopefully a more obvious requirement. I also updated the language slightly wherever we mention it.

Related issues

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