docs: add context doc

[a631807682]

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docs

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• This PR title match the format: <type>(optional scope): <description>
• The description of this PR title is user-oriented and clear enough for others to understand.
• Attach the PR updating the user documentation if the current PR requires user awareness at the usage level. User docs repo

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添加上下文文档

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en: add context doc
zh: 添加上下文文档
基于 #450 上添加的上下文文档 @li-jin-gou

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(optional) The PR that updates user documentation:

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Name	Status	Preview	Comments	Updated (UTC)
cloudwego	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jul 17, 2023 3:28am

[li-jin-gou]

thanks

[li-jin-gou]

看了一下没什么问题了，辛苦 @a631807682 @GuangmingLuo @welkeyever @rogerogers 再看下，没什么问题的话打算合了。

[li-jin-gou]

LGTM

[welkeyever]

@a631807682 @chaoranz758 High-quality output! Thank you very much for your contribution 🫡