docs: Refactor install docs a bit

[stephenh]

I.e. pull out the database setup into an optional/appendix, and just assume user's have a db setup.

Feedback:

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I didn’t use docker compose for my db. obv not required and makes setup seem harder than it is. I’d move it somewhere to keep the install process much more straight forward.

relatedly, I’d have the raw setup steps instead of a sample app. i.e. run npm init then npm i .. .

the “Installation” docs start with setting up docker, then setting up migration. before finally the first joist install. I think you want to start with joist and have the smallest steps to getting people writing joist code. could be more like: “run npm i joist…“, “we assume you have a postgres db so cut and paste this table schema into psql”, “run this to code gen the entities”, “write your first query”…

In my mind the next questions were: “how do I insert data” and “how do I query it”. I think providing some immediate examples of super simple CRUD operations would be good.