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.

開啟 https://beta.reactjs.org/learn/rendering-lists 中的 Rendering Lists 頁面，閱讀陷阱警告和「Where to get your key」區段。更新說明，使其區分索引 key 和隨機 key，並承認靜態列表是使用索引 key 的合理情況；當兩處的指引都清楚且一致時，即表示完成。