New API: remaining tasks and steps

[SimonHeybrock]

After #117, the following remains to be done:

• [x] Consider restoring handling of NXevent_data fields embedded in NXmonitor or NXdetector. This is used by SNS files.
• [x] Refactor/update docs. See #125
• [x] Release notes.
• [x] Release with new API in scippnexus.v2
• [x] Add deprecation warning in old API?
• [x] Make new API the default. See #158.
• [ ] Remove old API.
• [ ] Add tool for resolving depends_on chains.
• [ ] Refactor existing pieces for executing depends_on chains.
• [ ] Add tool for converting raw NXoff_geometry and NXcylindrical_geometry to per-detector "shapes".

[SimonHeybrock]

TODO: Move remaining tasks into issues.

[SimonHeybrock]

I created specific issues for the remaining tasks, closing.