md: Make exported markdown (from GDoc) match what you would hand-write

[googlecodelabs]

md: Make exported markdown (from GDoc) match what you would hand-write

[samtstern]

cc @kroikie

[samtstern]

For the code-blocks thing it looks like it depends on if it's a "terminal" block or not:

func (mw *mdWriter) code(n *types.CodeNode) {
	mw.newBlock()
	defer mw.writeBytes(newLine)
	if n.Term {
		var buf bytes.Buffer
		const prefix = "    "
		lineStart := true
		for _, r := range n.Value {
			if lineStart {
				buf.WriteString(prefix)
			}
			buf.WriteRune(r)
			lineStart = r == '\n'
		}
		mw.writeBytes(buf.Bytes())
		return
	}
	mw.writeString("```")
	mw.writeString(n.Lang)
	mw.writeBytes(newLine)
	mw.writeString(n.Value)
	if !mw.lineStart {
		mw.writeBytes(newLine)
	}
	mw.writeString("```")
}