Semigroup instance for MarkupContent is wrong

Markdown and plaintext have different semantics wrt newlines. The current semigroup instance simply coerces plaintext to markdown, but this breaks newlines. A more correct solution would be to do something like this for any plaintext arguments that are being transformed into markdown:

```
forceMarkdownNewlines :: String -> String
forceMarkdownNewlines = unlines . fmap (<> "  ") . lines
```