Add CSS Anchor Positioning Level 2 (#2138)

Close #2136, 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-anchor-position-2/",
  "seriesComposition": "delta",
  "shortname": "css-anchor-position-2",
  "series": {
    "shortname": "css-anchor-position",
    "currentSpecification": "css-anchor-position-1",
    "title": "CSS Anchor Positioning",
    "shortTitle": "CSS Anchor Positioning",
    "releaseUrl": "https://www.w3.org/TR/css-anchor-position/",
    "nightlyUrl": "https://drafts.csswg.org/css-anchor-position/"
  },
  "seriesVersion": "2",
  "seriesPrevious": "css-anchor-position-1",
  "organization": "W3C",
  "groups": [
    {
      "name": "Cascading Style Sheets (CSS) Working Group",
      "url": "https://www.w3.org/groups/wg/css/"
    }
  ],
  "nightly": {
    "url": "https://drafts.csswg.org/css-anchor-position-2/",
    "status": "Editor's Draft",
    "alternateUrls": [
      "https://w3c.github.io/csswg-drafts/css-anchor-position-2/"
    ],
    "repository": "https://github.com/w3c/csswg-drafts",
    "sourcePath": "css-anchor-position-2/Overview.bs",
    "filename": "index.html"
  },
  "title": "CSS Anchor Positioning Level 2",
  "source": "spec",
  "shortTitle": "CSS Anchor Positioning 2",
  "categories": [
    "browser"
  ],
  "standing": "good",
  "tests": {
    "repository": "https://github.com/web-platform-tests/wpt",
    "testPaths": [
      "css/css-anchor-position"
    ]
  }
}
```
</details>
<details><summary>Update spec in the same series (1)</summary>

```json
{
  "url": "https://www.w3.org/TR/css-anchor-position-1/",
  "seriesComposition": "full",
  "shortname": "css-anchor-position-1",
  "series": {
    "shortname": "css-anchor-position",
    "currentSpecification": "css-anchor-position-1",
    "title": "CSS Anchor Positioning",
    "shortTitle": "CSS Anchor Positioning",
    "releaseUrl": "https://www.w3.org/TR/css-anchor-position/",
    "nightlyUrl": "https://drafts.csswg.org/css-anchor-position/"
  },
  "seriesVersion": "1",
  "seriesNext": "css-anchor-position-2",
  "organization": "W3C",
  "groups": [
    {
      "name": "Cascading Style Sheets (CSS) Working Group",
      "url": "https://www.w3.org/groups/wg/css/"
    }
  ],
  "release": {
    "url": "https://www.w3.org/TR/css-anchor-position-1/",
    "status": "Working Draft",
    "filename": "Overview.html"
  },
  "nightly": {
    "url": "https://drafts.csswg.org/css-anchor-position-1/",
    "status": "Editor's Draft",
    "alternateUrls": [
      "https://w3c.github.io/csswg-drafts/css-anchor-position-1/",
      "https://w3c.github.io/csswg-drafts/css-anchor-position/"
    ],
    "repository": "https://github.com/w3c/csswg-drafts",
    "sourcePath": "css-anchor-position-1/Overview.bs",
    "filename": "index.html"
  },
  "title": "CSS Anchor Positioning",
  "source": "w3c",
  "shortTitle": "CSS Anchor Positioning",
  "categories": [
    "browser"
  ],
  "standing": "good",
  "tests": {
    "repository": "https://github.com/web-platform-tests/wpt",
    "testPaths": [
      "css/css-anchor-position"
    ]
  }
}
```
</details>

### Tests
These changes look good! 😎

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

Author: browser-specs-bot

specs.json

@@ -93,6 +93,7 @@
     "url": "https://drafts.css-houdini.org/font-metrics-api-1/",
     "standing": "good"
   },
+  "https://drafts.csswg.org/css-anchor-position-2/ delta",
   {
     "url": "https://drafts.csswg.org/css-backgrounds-4/",
     "seriesComposition": "delta",