Fixes #5788

Problem

Clicking a heading (e.g. "Coercion") on a docs page like /api sets the
URL to ?id=coercion. Clicking the same heading a second time causes the
page to jump to the top instead of staying at the section.

This happens because the <Link> only sets a query param (?id=coercion).
When the URL hasn't changed, Next.js treats it as a no-op navigation —
no scroll happens, and the page snaps to top via scroll restoration.

The right-side TOC nav already uses ?id=coercion#coercion (both query
param and hash), which works reliably on every click.

Fix

Add the hash to the heading anchor href to match what the TOC nav already does:

• Before: ?id=${props.id}
• After: ?id=${props.id}#${props.id}

This means every click is always a real URL change (the hash is always
present), and the browser's native anchor scrolling handles positioning
correctly and idempotently.