docs: improve Node adapter Fastify middleware error-page example

[sharadvc]

The Node adapter middleware docs show Fastify with app.use(ssrHandler), but @fastify/middie passes a next callback. When Astro does not match a route, it forwards to next() instead of rendering pages like 404.astro.

This adds a short explanation and a follow-up example that serves ./dist/client/ assets (including /_astro) while omitting next for other requests so Astro can render error pages.

Fixes #13864

[astrobot-houston]

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[ArmandPhilippot]

See #14541 (comment)