fix the toc for securing webhooks

[lyleschemmerling]

So in order to get that toc to populate needed to add the nested headings frontmatter param and export the headings from the sub component. Our implementation of the TOC expected some h2s and not only h3s so I changed those to make them populate and think that it looks okay, but am interested in what you think.

Related:

• https://docs.astro.build/en/core-concepts/layouts/#markdown-layout-props
• https://docs.astro.build/en/guides/markdown-content/#exported-properties