Use markdown-it plugin for toc

[muffinresearch]

Instead of using page data to build the toc it would be better to fix #659 and then use a toc plugin.

The same slugify function should be used everywhere for ids as that will make everything consistent:

Here's a sample of code that does the toc and heading id generation:

    [require('markdown-it-anchor'), {
      permalink: false,
      slugify: slugify
    }],
    [require('markdown-it-toc-done-right'), {
      listType: 'ol',
      level: 2,
      slugify: slugify,
      containerClass: 'table-of-contents-inner',
      callback: (html, ast) => {
        html = `<aside class="module-aside table-of-contents">${html}</aside>`;
      },
    }],

This should also then negate the need for doc authors working around issues with this by adding <section id="some-id-just-for-linkage">.