[API]: Add primary headings to Docs search responses

Product

Other/Multiple/Not Sure

Description

Documentation API search queries should include the top-level headings (often called the "table of contents" in Markdown parsers) with anchors and/or fully-formed direct URLs (as seen under the proposed headings key):

{
    "id": 826167,
    "title": "Application Configuration",
    "url": "https://craftcms.com/docs/5.x/reference/config/app",
    "uri": "docs/5.x/reference/config/app",
    "slug": "app",
    "type": "Craft CMS 5.x / Reference / Configuration Reference",
    "summaryPlain": "Craft’s entire application configuration can be customized via config/app.php. Any items returned by an app.php config file will get merged into the main application configuration array.",
    "summaryHtml": "Craft’s entire <a href=\"guide:structure-applications#application-configurations\">application configuration</a> can be customized via <code>config/app.php</code>. Any items returned by an <code>app.php</code> config file will get merged into the main application configuration array.",
    "headings": [
        { "level": 2, "name": "approach", "text": "Approach", "url": "https://craftcms.com/docs/5.x/reference/config/app#approach" },
        { "level": 2, "name": "requests-responses", "text": "Requests + Responses", "url": "https://craftcms.com/docs/5.x/reference/config/app#requests-responses" },
        { "level": 3, "name": "cors", "text": "CORS", "url": "https://craftcms.com/docs/5.x/reference/config/app#cors" }
    ]
}

We are currently testing the Heading Permalinks extension, and should be able to capture or generate all of this, as the text is rendered.

Start by tracing the Documentation API search response and the rendered Markdown headings, then review the Heading Permalinks extension referenced in the issue. Done means search results include a headings array with heading levels, names, text, and anchored or fully formed direct URLs.