docs: optimize cwgo overview and getting-started

[chaoranz758]

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docs

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en: optimize cwgo overview and getting-started zh(optional): 优化 cwgo 的概览和快速开始文档

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Name	Status	Preview	Comments	Updated (UTC)
cloudwego	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Feb 29, 2024 2:20pm