[Markdown] [Web/API] Make notes and warnings consistently formatted

[wbamberg]

This is part of #7898 .

The Markdown converter is a bit picky about what it considers to be valid notes and warnings.

Specifically, it expects to see:

• a <div> with class="note" or class="warning"
• whose first child is a <p>
• whose first child is <strong>Note:</strong> or <strong>Warning:</strong>

...e.g.

<div class="note">
  <p><strong>Note:</strong> ... </p>
</div>

or

<div class="warning">
  <p><strong>Warning:</strong> ... </p>
</div>

In Web/API, many notes and warnings don't follow this pattern (I counted about 800 notes, and I didn't count warnings yet, but probably not as many). These will need to be fixed.

Luckily there are a few common patterns we can automate (in particular <p><strong>Note</strong>:), and then there will be some leftovers we have to do manually.

[wbamberg]

#8244 completes this work :).