FEATURE: Add a TOC to a Post Header.

[P1R]

Motivation

We want that the TOC of a current POST (NOT PAGE) is shown at the top of the page-wrapper (viewed document ) and not in the Book-Summary.

Example like in a HackMD file when we add the following directive:

:::info
#### Table of Contents
[TOC]
:::

Img. 1. Shown example of what hackmd displays with that directive added to a document