docs: show a reliable way to preload fonts

[Nic-Polumeyv]

The docs say to opt fonts into preloading with a preload filter but not what to match against, so what everyone writes is a substring check against the hashed output path, which silently stops matching when the font is renamed. #16443 added filename for this; this shows it.

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Install the latest version of @sveltejs/kit from d31aec3:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/d31aec37ee04e7757aac488d3880f6dccab711a2

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16424

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[Rich-Harris]

Hmm. I wonder if we should provide a name property that represents the original basename. Only for font and asset types, since JS/CSS is bundled. We probably wouldn't even need a lookup, since assetFileNames is predictable

[Nic-Polumeyv]

Agreed, that would remove both footguns this recipe works around. Deriving it from the path mostly works, though there are edge cases I found: dotted basenames like inter.var.woff2, and rolldown sanitizing characters like + to _. We might not need to derive at all though, the client manifest already has the source path for every emitted asset and it's in scope in build_server.js where the fonts arrays get written.

I think I can implement this. I'm very supportive of this. Happy to make a separate PR and fix this one for docs.