docs: clarify gSSP docs for runtime

[leerob]

Replaces #65948.

[joulev]

I think the edge runtime should still be mentioned, just that it is experimental. For people who host on Cloudflare Pages and similar platforms, they must use the edge runtime even when it is experimental, so it is probably better that they know they can still do that with the Pages Router.

[leerob]

We don't have a hard and fast rule on documenting experimental features, but most of the time we do not. We could probably get more clarity on that. Further, we don't recommend using edge rendering, as we've now seen through many customers it can be slower than Node.js based rendering and a degraded DX having to juggle between two runtimes. While edge rendering was never made stable in the Pages Router, it is a stable option in the App Router. However, I would still recommend only using the edge runtime for Middleware and not for rendering.