`filewriter`: specify file layout in detail

We need to formally document what we expect any 'new' `.nxs` files to look like, and pay particular attention to differences with the old system.

The goal is to be compatible where possible / where it makes sense, but some information may fundamentally differ between ICP and streaming stack, e.g.:
- Much of the information in the `isis_vms_compat` group
- How we write per-frame veto state and enablement alongside the event-mode data
- Various metadata fields
- Some pieces of information which only make sense in ICP (or in the newer streaming system)

**This ticket must include discussion with stakeholders such as DSG and Mantid (and other relevant groups)**

## Acceptance criteria

- [ ] Documented structure for what a 'new' nexus file will look like
- [ ] Documentation describing differences where they exist (effectively a 'porting guide' between ICP and streaming files)
- [ ] An example file in the new format (does not need to have been written by 'new' filewriter; e.g. a simple python program can be used to generate this example file / manipulate an existing file)