[web-php] PR #804: Add redirect for yield keyword

Pull Request: https://github.com/php/web-php/pull/804 Author: fredden When browsing to https://php.net/yield recently, I was surprised to see a message saying that "yield doesn't exist". The yield keyword does exist, but doesn't have a dedicated page so hasn't been indexed. This pull request adds a special case for this keyword so that it can be found by users. I've also looked into how to get all keywords declared automatically. The best I can come up with today (without significant changes to how the search / look-up process works) is over on the 'systems' repository: https://github.com/php/systems/pull/19