docs: update readme for structures pruning scripts

The structures pruning script is little known, but rather useful.
Update its readme with some quick information,
and a link to more detailed information on the official docs site.

Also convert the readme to markdown, so we can use the Github alerts markdown extension: https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#alerts .

Depends-on: https://github.com/openedx/docs.openedx.org/pull/1496

Private-ref: https://tasks.opencraft.com/browse/BB-11072