docs: use open sans font and update slogan

[chenjiahan]

Summary

• Use open sans font in the website.
• Update slogan, emphasize "Lightning Fast" instead of "Rspack-based".

before:

after:

Checklist

• [ ] Tests updated (or not required).
• [x] Documentation updated (or not required).

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[Huxpro]

Why do we pick Open Sans over the current font?

Personally I feel like the letter-spacing for Open Sans is too loose

[chenjiahan]

The Node.js new website uses Open Sans and we like it 😄