Document image captions support

[osulyanov]

Documents Add image captions support.

Example Syntax

![Alt text](_images/image.png "Caption text" =100x100){ caption }

In this example, the "Caption text" should be rendered within a figcaption tag below the image.

Example HTML Output

The resulting HTML should look like this:

<figure>
    <img src="_images/image.png" alt="Alt text" width="100" height="100" title="Caption text">
    <figcaption>Caption text</figcaption>
</figure>

DOCSTOOLS-3428

Related issue: diplodoc-platform/transform#514