docbook: richer parse for admonitions (#5593)

Fixes #1234.

This parses admonitions not as a blockquote, but rather as a div with an
appropriate class. We also handle titles for admonitions as a nested div
with the "title" class.

(I followed the behaviour of other docbook-to-html converters in this -
there are clearly other ways you could encode it.)

In general, the handling of elements with nested title elements is very
inconsistent. I think we should make it consistent, but I'm leaivng that
for later to make this a small change.

Example:
```docbook
<warning xml:id="someId">
<title>My title</title>
<simpara>An admonition block</simpara>
</warning>
```

goes to

```html
<div id="someId" class="warning">
<div class="title">My title</div>
<p>An admonition block</p>
</div>
```

0 files changed, 0 insertions, 0 deletions