fix(www): make mdx h2 id generation handle nested heading content

[zigzagdev]

Description

The h2 heading id was built with props.children?.toString(), which returns "[object Object]" whenever a heading contains nested markdown like bold or code, breaking the anchor id and any deep link or table-of-contents jump to that heading.
It also only stripped spaces, apostrophes, and question marks, leaving other punctuation (:, ,, (, ), /) in the generated id.
This PR walks the children tree to extract plain text first, then slugifies it properly.

Changes

• Add getHeadingText() to recursively extract plain text from a heading's React children tree
• Add slugifyHeading() to normalize that text into a safe, predictable anchor id (lowercase, strip non-alphanumeric characters, collapse whitespace to hyphens)
• Use both in the h2 MDX component's id generation instead of props.children?.toString()

Motivation

At least two live blog posts have h2 headings wrapped in bold, which rendered their anchor id as the literal string object-object, breaking deep links and table-of-contents navigation to those sections.
Headings with other punctuation (colons, parentheses, commas, slashes) — 66 across docs/blog — also produced invalid or unpredictable ids.
So, this fix makes heading id generation robust regardless of inline formatting.

Breaking Changes

None

Screenshots

Device / Browser / Viewport:

Before	After

Not applicable

Checklist

• pnpm check passes locally
• pnpm build passes locally
• pnpm build:registry was run and the generated files are committed (if you changed registry/ or config/site.ts — CI verifies this on every PR)
• PR title follows Conventional Commits (e.g., feat(marquee): add reverse prop)

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[Yeom-JinHo]

Thanks @zigzagdev! Closing since this doesn't reproduce: fumadocs' remarkHeading already sets heading ids, and {...props} is spread after id so the custom id is never applied (prod renders e.g. the-benefits-of-using-a-react-ui-component-library).