Explain index as key can be ok

The pitfall warning on https://beta.reactjs.org/learn/rendering-lists states that using index or random for keys is bad. However, this is the only mention of using indexes of keys. I think it should state that using indexes is bad, but it should also be clear that using random values much worse.

The problem is using index as key is a well known anti-pattern in the React world. People come up with much worse solutions, e.g. use a random key, to get rid of ESLint warnings.

Also there are legitimate use cases for an index as a key, for example when rendering a static list of data. I think this should be acknowledged in the Where to get your key section.

打开 https://beta.reactjs.org/learn/rendering-lists 中的 Rendering Lists 页面，阅读陷阱警告和“Where to get your key”部分。更新说明，使其区分索引 key 和随机 key，并承认静态列表是使用索引 key 的合理情况；当两处的指导都清晰且一致时，即表示完成。