docs: More improve optimization chapter for SSR

[kazupon]

Hi!
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When vue-i18n need SSR build, we can support for it with @intlify/unplugin-vue-i18n and ssr option.
https://github.com/intlify/bundle-tools/tree/main/packages/unplugin-vue-i18n#ssr

But, I have realized vue-i18n docs is poor about it.
About SSR configuration, Optimization chapter should be written it.
https://vue-i18n.intlify.dev/guide/advanced/optimization.html#optimization

Originally posted by @kazupon in #2253