Close #1549, adding the suggested spec to the list.

### Changes to `index.json`
This update would trigger the following changes in `index.json`:

<details><summary>Add spec (1)</summary>

```json
{
  "url": "https://wicg.github.io/web_audio_playout/",
  "seriesComposition": "full",
  "shortname": "web_audio_playout",
  "series": {
    "shortname": "web_audio_playout",
    "currentSpecification": "web_audio_playout",
    "title": "Playout Statistics API for WebAudio",
    "shortTitle": "Playout Statistics API for WebAudio",
    "nightlyUrl": "https://wicg.github.io/web_audio_playout/"
  },
  "standing": "pending",
  "organization": "W3C",
  "groups": [
    {
      "name": "Web Platform Incubator Community Group",
      "url": "https://www.w3.org/community/wicg/"
    }
  ],
  "nightly": {
    "url": "https://wicg.github.io/web_audio_playout/",
    "status": "Unofficial Proposal Draft",
    "alternateUrls": [],
    "repository": "https://github.com/WICG/web_audio_playout",
    "sourcePath": "index.html",
    "filename": "index.html"
  },
  "title": "Playout Statistics API for WebAudio",
  "source": "spec",
  "shortTitle": "Playout Statistics API for WebAudio",
  "categories": [
    "browser"
  ]
}
```
</details>

### Tests
These changes look good! 😎