message: feat(build): parse heading for textContent before slugifying
  In some cases, we are rendering headings that contain inline html
  instead of just text, this parses headings during build using JSDOM
  and uses the parsed textContent as the hash instead
diff: +import { JSDOM } from 'jsdom';
  import { slugify } from '../../../src/utils';
  export default (text: string, level: number) => {
  - const hash = slugify(text);
  + const { textContent } = JSDOM.fragment(text);
  + const hash = slugify(textContent);
  return `
  <h${level} id="${hash}">
  <a href="#${hash}">${text}</a>
  