JS redirect solution for anchor links

When a section title is changed it breaks anchor links.
(Note, this has to do with the actual title of the section, not the in=sphinx section reference name.)

We can't fix this server side (anchor links are handled on the client).
It would be good to have some kind of anchor link redirect feature,
with a simple mapping file that content authors can update if they re-title a section.

Start by locating the client-side anchor-link handling in the Sphinx-built documentation and determine where a mapping file could be loaded. Define the redirect behavior for renamed section-title anchors and verify that old links reach the corresponding sections.

Written by the indexing model from the issue text.