doc: add a few tips on converting from match-style assertions

[Kazark]

With this in place, I think MUnit's documentation captures everything I learned during the conversion. I wonder, though, whether it would be worthwhile to make this page a little more self-contained, i.e. what about linking back to other parts of the documentation that are relevant? Part of the reason I say this is because when a teammate of mine original saw this page, he remarked that it was rather sparse. Perhaps it would be nice to be able to have people go straight to a page on converting, and from there be guided to the other parts of the documentation that they should read. If folks like this idea, I can submit another PR against this page with links to other part of the documentation that I found especially necessary during the conversion.

[olafurpg]

I think it’s a great idea to make the scalatest migration page more self contained! I agree the current page is quite sparse. Thank you 🙏