xv83 cleanup

We need to clean up project xv83 on NCI so that it follows the [desired directory structure](https://github.com/AusClimateService/data-code-group/blob/main/data_roadmap.md#working-space-xv83-and-mn51). This issue contains the to-do list for the clean up.

- [ ] Create `/g/data/xv83/README` to explain the basic directory structure (Damien Irving)

Move user directories into `/g/data/xv83/users/` (Damien to contact individuals):
- [ ] Chloe Mackallah
- [ ] Doug Richardson
- [ ] Jozef Syktus
- [ ] John Taylor
- [ ] Marcus Thatcher
- [ ] Rebecca Gregory
- [ ] Richard Matear
- [ ] Stacey Osbrough
- [ ] Benjamin Ng
- [ ] Damien Irving
- [ ] Didier Monselesan
- [ ] Jyoteeshkumar reddy Papari
- [ ] Kavina Dayal
- [ ] Nicholas Pittman
- [ ] Raktima Dey (get ownership transferred)

Move datasets to `/g/data/xv83/datasets/` (or possibly `/g/data/ia39/aus-ref-clim-data-nci/`):
- [ ] `agcd-csiro` (Damien Irving)
- [ ] `dcfp` (Dougie Squire)
- [x] `from_ua6` (large ensembles; Claire Trenham)
- [x] `shapefiles` (Damien Irving)

Assess whether to remove or move:
- [ ] `/g/data/xv83/observations/CCI_fire/` (Dougie Squire)
- [ ] `/g/data/xv83/reanalyses/` (Dougie Squire)