I notice that you say that the "Installation" section should "cover the architectural overview and point people to where they can find the stuff they need to install PostgreSQL in the various ways available to them" so maybe you're not imagining a four-sentence chapter, either.
Fair point but I posit that new users are looking for a chapter named Installation in the documentation. At least the ones willing to read documentation. Having two of them isn't needed but having zero doesn't make sense either.
The current proposal does that so I'm ok as-is but it can be further improved by moving source install talk elsewhere and having the installation chapter redirect the reader there for details. I'm not concerned with how long or short the resultant installation chapter is.
