Positive / Negative Codelab formatting

[Scott3142]

There is a known issue on the Google side that formats "positive" and "negative" blocks incorrectly.

Track the progress of this issue here: https://groups.google.com/g/codelab-authors/c/wgzDYw5VYzg

There is a workaround which suggests changing the markdown from

Positive
: Notes

to

<aside class="positive">
Notes
</aside>

(and likewise for negative), but on first glance this appears to strip out other md formatting. This will not be fixed in these notes - we will await a fix from Google.