feat: add src param and display categories for faq shortcode

The src param can target data files for the FAQs.

This also displays multi-level headings (only 2 supported for now...).

I am using the category property from https://schema.org/category.

It doesn't seem like this property does anything for Question (https://schema.org/Question). But it was the closest thing I could find to indicate sections on a FAQ page.