Add CSS Gap Decorations Module Level 1

Close #1598, 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://drafts.csswg.org/css-gaps-1/",
  "seriesComposition": "full",
  "shortname": "css-gaps-1",
  "series": {
    "shortname": "css-gaps",
    "currentSpecification": "css-gaps-1",
    "title": "CSS Gap Decorations",
    "shortTitle": "CSS Gap Decorations",
    "nightlyUrl": "https://drafts.csswg.org/css-gaps/"
  },
  "seriesVersion": "1",
  "organization": "W3C",
  "groups": [
    {
      "name": "Cascading Style Sheets (CSS) Working Group",
      "url": "https://www.w3.org/Style/CSS/"
    }
  ],
  "nightly": {
    "url": "https://drafts.csswg.org/css-gaps-1/",
    "status": "Editor's Draft",
    "alternateUrls": [
      "https://w3c.github.io/csswg-drafts/css-gaps-1/",
      "https://w3c.github.io/csswg-drafts/css-gaps/"
    ],
    "repository": "https://github.com/w3c/csswg-drafts",
    "sourcePath": "css-gaps-1/Overview.bs",
    "filename": "index.html"
  },
  "title": "CSS Gap Decorations Module Level 1",
  "source": "spec",
  "shortTitle": "CSS Gap Decorations 1",
  "categories": [
    "browser"
  ],
  "standing": "good"
}
```
</details>

### Tests
These changes look good! 😎

specs.json

@@ -104,6 +104,7 @@
   "https://drafts.csswg.org/css-env-1/",
   "https://drafts.csswg.org/css-extensions-1/",
   "https://drafts.csswg.org/css-forms-1/",
+  "https://drafts.csswg.org/css-gaps-1/",
   {
     "url": "https://drafts.csswg.org/css-gcpm-4/",
     "seriesComposition": "delta",