Rewrite Async 101

[AndrewDiMola]

The introduction should be shorter, with less (or smaller-sized) diagrams.

Conceptually, it's important to describe how the asynchronous functionality actually works and potential misconceptions, but I don't think the first overview page is an appropriate place to go into all of that information in depth.

The Awaitables page is more accessible and often seems like the better 101 than the actual 101 page.

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Please don't change anything below this point.

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• Build ID: HHVM=HHVM-4.164.0:HSL=v4.108.1:2022-07-26T17:24:38+0000:1fa47f258c6b68f8ec01899aa82fd6ffa0957109
• Page requested: /hack/asynchronous-operations/introduction
• Page requested at: Mon, 08 Aug 2022 15:22:39 +0000