docs: Add documentation for heading and table of contents

[fsbraun]

Is your feature request related to a problem? Please describe.
The usage of the heading and table of content plugins is not intuitive. TOC entries are only created upon rendering of heading plugins, and only if they have an anchor defined.

This means the table of contents can only be produced below all heading plugins. To display it at the to of a page, grid features are needed, e.g., a separate column which contains the toc to be rendered at the end of a page but which is displayed at the top.

Describe the solution you'd like
Add a document for headings and toc to the documentation.