chore: mark markdown output from process-messages as generated

[Conduitry]

We're marking the generated js files as generated; it would be nice to mark the markdown files as well.

Hopefully this doesn't confuse the site when it gets consumed. I haven't tested that.

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[changeset-bot]

⚠️ No Changeset found

Latest commit: 3c3e566

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[pkg-pr-new]

pnpm add https://pkg.pr.new/svelte@13913

commit: 3c3e566

[dummdidumm]

They're already in a .generated folder, is that not enough for you?

[Conduitry]

I don't have an especially strong opinion about this. This PR arose from me doing a global search in the repo for ... something, I no longer remember what. And when I was looking at these files, I didn't actually notice that they had .generated in the path. I just noticed that there were several search results for what I was looking for, and some of the files I shouldn't be editing had a note at the top of them telling me that, and other files I shouldn't be editing did not.

[Rich-Harris]

seems reasonable