feat: use hyphens

This makes the FAQ headings have IDs with hyphens. This is more in line with how Hugo handles the IDs for headings created in Markdown.

This shouldn't break any existing links - I don't believe this shortcode is used anywhere.

I tried to remove the TOC from this shortcode in favour of the table_of_contents shortcode we recently added to the theme. However, shortcode HTML headings are not added to Hugo's built-in TOC data structure. So, we still need to keep this duplicate code.