Page-level table of contents

[jsonmaur]

Support page-level TOC, similar to how Bugsnag does it. We would need to create a custom component, or somehow give the user the ability to render the TOC where they want in the page.

[jsonmaur]

Decided to go with adding a front matter item to enable a TOC. The theme is responsible for creating the look and feel of it. From the user perspective, they will add this to their front matter:

---
toc: prepend
// or
toc: append
---

Which will add links to the page for each sub-item. You can also add a description front matter item to the page itself to give the toc a description as well as a title.

[jsonmaur]

Updated to use table_of_contents front matter key for both page-level TOC and folder-level TOC.

---
table_of_contents:
  page: false
  folder: true
---

or in config file:

"table_of_contents": {
  "page": false,
  "folder": true
}

[jsonmaur]

Implemented in #140.