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.

Open the Rendering Lists page at https://beta.reactjs.org/learn/rendering-lists and read the pitfall warning and the “Where to get your key” section. Update the explanation so it distinguishes index keys from random keys and acknowledges static lists as a legitimate index-key case; done means the guidance is clear and consistent in both places.