Table of Content Should Use Header "id"s If Possible

[Zehua-Chen]

During the table of content generation, the current implementation would use the content of the header, transform it a little bit, and then use the transformed as the id of the header and the ref of the table of content entries.

For non-ASCII headers that have custom ids, this approach proves problematic, as no ID will be generated from the non-ASCII header, resulting in a broken table of content.

# 注意事项 {#notes}

To fix this, I think the ID of headers should be used if they are available, and we only fall back to header content otherwise.

[stale]

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