Document dataset deletion

While https://github.com/assemblee-virtuelle/semapps/issues/871 is pending, we should add a "Deleting datasets" page to the triple store documentation (https://semapps.org/docs/triplestore/)

Start with the triple store documentation at https://semapps.org/docs/triplestore/ and read issue #871 to understand the pending dataset-deletion behavior. Add a “Deleting datasets” page there that documents the supported process and make the page discoverable from the surrounding documentation.

Written by the indexing model from the issue text.