Add AT Driver

Close #1519, 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://w3c.github.io/at-driver/",
  "seriesComposition": "full",
  "shortname": "at-driver",
  "series": {
    "shortname": "at-driver",
    "currentSpecification": "at-driver",
    "title": "AT Driver",
    "shortTitle": "AT Driver",
    "nightlyUrl": "https://w3c.github.io/at-driver/"
  },
  "organization": "W3C",
  "groups": [
    {
      "name": "Browser Testing and Tools Working Group",
      "url": "https://www.w3.org/groups/wg/browser-tools-testing/"
    }
  ],
  "nightly": {
    "url": "https://w3c.github.io/at-driver/",
    "status": "Editor's Draft",
    "alternateUrls": [],
    "repository": "https://github.com/w3c/at-driver",
    "sourcePath": "index.bs",
    "filename": "index.html"
  },
  "title": "AT Driver",
  "source": "spec",
  "shortTitle": "AT Driver",
  "categories": [
    "browser"
  ],
  "standing": "good"
}
```
</details>

### Tests
These changes look good! 😎

specs.json

@@ -497,6 +497,7 @@
   "https://testutils.spec.whatwg.org/",
   "https://url.spec.whatwg.org/",
   "https://urlpattern.spec.whatwg.org/",
+  "https://w3c.github.io/at-driver/",
   "https://w3c.github.io/audio-session/",
   "https://w3c.github.io/contentEditable/",
   {