mini-epic: docx validation

[cscheid]

When Pandoc ships a release with all the docx validation issues resolved (see eg jgm/pandoc#9273), we should do the same for our outputs.

• Run a docx validator ahead of providing output #6365

[cderv]

For reference, related issues where this is discussed

• Run a docx validator ahead of providing output #6365

Pandoc CI has now a docx validator run in CI: https://github.com/jgm/pandoc/blob/main/.github/workflows/docx-validation.yaml
We could do an adapted CI on our side.

[cderv]

Pandoc 3.11.1.1 is the one that validates output: https://pandoc.org/releases.html#pandoc-3.1.11.1-2023-01-05

Docx writer:

Ensure that pandoc’s output validates (Edwin Török, #9273, #9269, John MacFarlane, #9265, #9266, #9264).

[cderv]

For reference, related issues:

• Run a docx validator ahead of providing output #6365

[edwintorok]

#8392 I've opened a PR here with some validation fixes (for 1.3).
Haven't tried 1.4 yet (IIRC last I tried it had more errors than 1.3), and I likely won't have time in the short term to port the fix to 1.4/main, but if anyone else wants to try I am happy to help/explain what any new validation errors mean and how to fix them.