docs: clarify that SSR is required for SSG with content (#14471)

[th0rgall]

See issue #14471. My main edit here is in the adapter-static (SSG) docs, where I believe a warning should be placed to not disable SSR when trying to achieve SSG.

I also added a few warnings and links in the glossary and page options docs to guide people to the right information when they are researching SSG in SvelteKit.

These changes don't solve the situation that it's conceptually counter-intuitive (to me) that SSG/prerendering requires "SSR", but it tries to help developers to not making unintended mistakes based on wrong assumptions.

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