Ambiguous "Overview" link text on the documentation home page

[Natalia Bidart]

The ​documentation home page uses Overview as the link text 15 times. It is the only duplicated link text on the page. A sighted reader sees which group each one belongs to; someone navigating by a list of links gets 15 identical entries. Most of this is self-inflicted. Every one of those targets already has a distinct title, which docs/index.txt overrides with the generic word.

For most of them the fix is to remove the explicit title and let :doc: use the document's own. Where that would repeat the group label, a short descriptive title does the job. While fixing the link text, a few groups are worth tidying: single-link groups whose label repeats their only link, and About this documentation, which is already reachable under Contributing to Django.

This is a derived from this ​forum post, following ticket #37051.