Mainly:
- Try to triangulate between narrative that framed this as
a new/easy process and the need for a reference that will
not quickly grow stale.
- Fix a ~continuity issue where the text was talking about
"your Cachix cache" before saying that you'd need to make
a Cachix cache to enable the installer tests.
- Adopt suggestion on titling, and nest subtitles in the
installer test section.
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