[Documentation] Quick start remind user to run Postgres server beforehand

[badmonster0]

Quickstart docs, it's helpful to link the reader to the "launch cocoindex server" before running cocoindex update main. For new users, they may forget to run the Postgres server beforehand (like I did)

[Haleshot]

I can add a suitable admonition in the Quickstart doc webpage pointing to Configure CocoIndex Settings > two ways to launch Cocoindex.

[badmonster0]

it's yours. @Haleshot thanks for taking it!

cc @georgeh0