docs: add Rstest to homepage

[chenjiahan]

Summary

• Update @rstack-dev/doc-ui to 1.9.0 to add Rstest to homepage.
• This update also includes some UI and text improvements.

Related: https://github.com/rspack-contrib/rstack-doc-ui/releases/tag/v1.9.0

Before

After

Checklist

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

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[fi3ework]

It feels a bit personal, but do we really need to use a strong red color for the documentation link? It gives off an "error" vibe, even though it's very eye-catching. 🤔

[chenjiahan]

This is one of our brand colors that we have used in many places, such as:

• Our home button

• Nuxt homepage

I’m open to using other colors though, do you have any color recommendations that fit the brand? 😄

[fi3ework]

In the second image, rspack.dev has a very highly saturated red, especially compared to the other two links (vue and nitro). I'm not a design professional, but I believe a lower saturation color would work better. Never mind, we can ask the team's designer for suggestions. 😀

[chenjiahan]

Reduced contrast in rspack-contrib/rstack-doc-ui#37