fix!: html validation: Do not export <style> elements above TOC

[kaushalmodi]

<style> elements are not allowed inside

HTML elements. It is
very likely that the TOC + post content would be wrapped in an
<article> element, and in those cases, the HTML Validator will throw
an error like:

Element style not allowed as child of element article in this
context.

The breaking change will be seen only when both toc and section
numbering is enabled. The 'breakage' is visual -- that the numbered
headings in the TOCs will have the unordered list bullets.

To fix it, add this to the CSS:

.toc.has-section-numbers ul {
  list-style: none;
}