Use HTMLHeading elements instead of Markdown.

Given the following GitLab Flavored Markdown (GLFM):

 <h2>Table of Contents</h2>

The HTMLHeading element `<h2>` will not appear in GLFM's \[TOC\].

[TOC]

## Visible Heading 2 appears in the \[TOC\].

### Visible Heading 3 also appears in the \[TOC\].

<h4>This HTMLHeading element will _not_ appear in the \[TOC\].</h4>

<h4>This HTMLHeading element will _not_ appear in the \[TOC\], either.</h4>

### Visible Heading will appear.

Gitlab will render: