[javascript] Add generators

Generators (`function*` and `yield`) are not mentioned in the current version.

The issue does not name a file, so first locate the JavaScript guide/page in this docs repo (indicated by the `[javascript]` tag). Read that document to find where language features are documented, then add a generators section covering `function*` and `yield` in the same style as nearby topics. Validate by reviewing the updated markdown content for consistency and checking the rendered docs flow used by contributors; done means generators are clearly documented in the JS guide.

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