[Standards] Spec proposal: bookmarks hierarchy

Hello all, Edhelas and I submitted a PR on github for a spec proposal: <https://github.com/xsf/xeps/pull/1574> A rendered version is at <https://nicoco.fr/bookmarks-hierarchy.htm>. The idea is to have an additional item in pubsub/pep nodes containing bookmark items, defining bookmarks directories. The bookmark items themselves have a dedicated extension referring to these directories, with a "weight" used to sort them within the categories. Our obvious use case is organising rooms in a XEP-0503 space, but it could possibly be used for organising PEP bookmarks too. Feedback welcome and appreciated! Have a nice sunday, -- nicoco