Extend toctree to avoid duplication of links in the section landing pages

The TocTree render is currently limited to a list of links.

In order to have a render with maybe a small introduction, the list of links must be duplicated.

Extending toctree to be able to have a nicer render would avoid link duplication and simplify maintenance.

Start by locating the TocTree render and the section landing pages that currently duplicate their links. Read how the existing list-only render is assembled and determine how an introduction should be represented. Done means landing pages can include a small introduction without duplicating the toctree links, with the documentation render verifying the result.

Written by the indexing model from the issue text.