Make markdown formatting requirements explicit

[bielsnohr]

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Talking to @tobyhodges at CarpentryConnect last year, we found out that if we ever want to translate in the future, we need to keep sentences all on one line. This unfortunately goes against all of the SemBr work that was done. Luckily, we don't explicitly say anywhere that we are enforcing SemBr, but we should be explicit about how the markdown we use for lessons should be formatted. I'm not sure whether this could be captured in something like markdownlint (i.e. a linter) but at the very least we should write down somewhere that:

1. All sentences should be on a single line
2. Therefore, it doesn't matter how long a line is and it shouldn't be broken

In some respects, this is a bit of a shame because it will make editing Markdown files not as easy, which was part of the reason we made a push to semantic line breaks in the first place. But I feel like the potential for translating is probably more important.

Thoughts?