Add normative-conventions.md; document agreed-upon coercion rules

[bakkot]

We got explicit consensus for these rules in the July 2023, September 2023, November 2023, and April 2024 meetings (notes not yet available for the April 2024).

https://github.com/tc39/how-we-work/pull/119 would introduce a spec-conventions.md. I think we shouldn't mix normative with editorial conventions, so I've made a different document. But the sole normative convention currently in that PR would be good to include in this new document.