Embedded video — ou-video admonition

Embed a video player capable of playing of a specified video file:

```{ou-video} resources/test.mp4
```

We can also include one or more than one line of text in the body of the admonition.

```{ou-video} resources/test.mp4
A caption for a video file.

A line of description.
And continuation of the line.

More description.
```

TO DO - class and style the caption etc?

The first line is mapped to a caption. Any additional lines are mapped to a description. The corresponding OU-XML is:

<MediaContent type="video" height="" width="" src="https://raw.githubusercontent.com/innovationoutside/sphinxcontrib-ou-xml-tags/main/vletmp/ouseful-demo-sphinx_b0_p1_x_media_test.mp4">
    <Caption>Figure 2.1 A caption for a video file.</Caption>
    <Description>
        <Paragraph>A line of description.
            And continuation of the line.</Paragraph>
        <Paragraph>More description.</Paragraph>
    </Description>
</MediaContent>

The MyST spec also lets you use a video file path in a {figure} admonition when generating HTML output, but this is not (yet?) supported for conversion to OU-XML. There is a also a sphinx-contrib/youtube extension for embedding videos, but again there is no direct path for generating an appropriate OU-XML media tag.

TO DO: how does OU-XML recommend YouTube embeds?

TO DO: support MyST generated video embeds in conversion to OU-XML.