Support ANSI sequences in `index.md`

[krlmlr]

They are being stripped out immediately after converting Markdown to HTML. This works for me in duckplyr: https://github.com/tidyverse/duckplyr/actions/runs/11086444293/job/30803979684 is built using this branch, the result is in https://duckplyr.tidyverse.org/ .

Not sure how to add tests here.

[github-actions[bot]]

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[maelle]

@krlmlr I added two tests that fail on the main branch. :smile_cat:

[krlmlr]

Done.