docs: head config only takes effect in production mode

[9aoy]

Summary

update docs, head config only takes effect in production mode.

Related Issue

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[chenjiahan]

Can we make it work in dev?

[9aoy]

Can we make it work in dev?

It would be better if dev could support it.

[Timeless0911]

See #1432

I think we can redesign header config in Rspress 2.0 to make it work well in both dev and production mode. Maybe we can directly compose and pass header-related config in config/fronmatter to html.tags config in Rsbuild.