DEVSITE-1954: Fixed the no url update bug in local dev

[jiangy10]

Problem

Anchor links are empty when running in local development environment. When clicking on a header link or an "On This Page" navigation link, the URL contains an empty anchor (e.g., # with nothing after it).

Reason

The to-hast.ts step converts MDAST (Markdown Abstract Syntax Tree) to HAST (HTML Abstract Syntax Tree). Previously, it used defaultHandlers from mdast-util-to-hast, which does not generate id attributes for heading elements.
In local development, content is processed directly through the runtime connector without this additional step, resulting in headings without IDs.

Solution

Added a custom heading handler that:

• Extracts the text content from the heading using hast-util-to-string
• Converts the text to a URL-friendly slug (lowercase, hyphenated)
• Sets the generated slug as the id property on the heading element

Result

• Before: <h2>anchor</h2>
• After: <h2 id="anchor">anchor</h2>