Add a table of contents to all pages, similar to Wikipedia

[coenkoopmans]

Description

On Wikipedia, almost every page has a table of contents, like this one: https://en.wikipedia.org/wiki/JavaScript

The strong points of a table of contents are:

• Quick overview of the contents of the page
• Easy navigation to the specific part that interests the reader

This is perfect for a reference work, such as our documentation. For these reasons, we should build a table of contents of our own that is easy to integrate into the MD files (almost all of them, context dependent). That means finding a way that is easily reusable and wards against errors when consuming the code in the MD files.

Acceptance criteria

• [ ] Create a reusable index component for use in MD files
• [ ] It also works on GitHub (when viewing a consuming MD file in GitHub)
• [ ] Clicking on index links brings you to corresponding headings based on HTML element ID's.
• [ ] All pages with multiple headings have this index implemented

[jordybulten]

As a POC we updated the App Layout component already. For now we should update the Contributing page to explain how to create a page with a TOC. See #56