[docs] adapter-static: document prerender.default and info about disabling ssr in dev

[Theo-Steiner]

#4192 introduced a configuration setting for prerendering by default, which explicitly has to be set to true for adapter-static to work in non-SPA mode. Currently adapter-static only tells the user to do so when actually building, but having it in the docs is good for obvious reasons.
I also lumped in a small mention about disabling ssr in the handle() hook, to make developing an SPA with svelte-kit aligned with how it will actually works when built. I hope it's okay that I made a single PR for two changes, just let me know if changes are required!

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[Rich-Harris]

thanks!