Add Facilitated Payment Link Type in HTML (#1676)

Close #1674, 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/paymentlink/",
  "seriesComposition": "full",
  "shortname": "paymentlink",
  "series": {
    "shortname": "paymentlink",
    "currentSpecification": "paymentlink",
    "title": "Facilitated Payment Link Type in HTML",
    "shortTitle": "Facilitated Payment Link Type in HTML",
    "nightlyUrl": "https://wicg.github.io/paymentlink/"
  },
  "organization": "W3C",
  "groups": [
    {
      "name": "Web Platform Incubator Community Group",
      "url": "https://www.w3.org/community/wicg/"
    }
  ],
  "nightly": {
    "url": "https://wicg.github.io/paymentlink/",
    "status": "Draft Community Group Report",
    "alternateUrls": [],
    "repository": "https://github.com/WICG/paymentlink",
    "sourcePath": "index.bs",
    "filename": "index.html"
  },
  "title": "Facilitated Payment Link Type in HTML",
  "source": "spec",
  "shortTitle": "Facilitated Payment Link Type in HTML",
  "categories": [
    "browser"
  ],
  "standing": "good"
}
```
</details>

### Tests
These changes look good! 😎

Co-authored-by: browser-specs-bot <>

specs.json

@@ -731,6 +731,7 @@
   "https://wicg.github.io/observable/",
   "https://wicg.github.io/overscroll-scrollend-events/",
   "https://wicg.github.io/page-lifecycle/",
+  "https://wicg.github.io/paymentlink/",
   "https://wicg.github.io/performance-measure-memory/",
   "https://wicg.github.io/periodic-background-sync/",
   "https://wicg.github.io/permissions-request/",