GHSA-5r3x-p7xx-x6q5

### Impact A Comrak AST can be constructed manually by a program instead of parsing a Markdown document with `parse_document`. This AST can then be converted to HTML via `html::format_document_with_plugins`. However, the HTML formatting code assumes that the AST is well-formed. For example, many AST notes contain `[u8]` fields which the formatting code assumes is valid UTF-8 data. Several bugs can be triggered if this is not the case. ### Patches 0.17.0 contains adjustments to the AST, storing strings instead of unvalidated byte arrays. ### Workarounds * Validate UTF-8 correctness of all data when assigning to `&[u8]` and `Vec<u8>` fields in the AST. ### References n/a