Support separate alt text and captions in Markdown content

[conradolandia]

This PR replaces a plugin to add support for proper Alt text from markdown sources using mdsvex. The new plugin is rehype-title-figure.

With this new implementation, ![This is alt text](image.jpg "This is the caption") will render like <figure><img src="image.jpg" alt="This is alt text" title="This is the caption"/><figcaption>This is the caption</figcaption></figure>.

An image without a caption will not produce a figure nor figcaption, but will produce an img with an alt attribute: ![This is alt text](image.jpg) will render like <img src="image.jpg" alt="This is alt text" />.

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