feat: add figure shortcode and documentation

feat: add figure shortcode and documentation

Ported figure shortcode from previous theme. Wraps content in semantic <figure> element with optional title, caption, attribution, and link support.

Typical usage:
{{< figure caption="Image caption" attr="Photo by Someone" >}}
{{< img src="image.jpg" alt="Description" >}}
{{< /figure >}}

Also updated CONTENT_GUIDE with comprehensive usage examples and parameters.

Co-Authored-By: Claude Haiku 4.5 <redacted>