Add NWB-specific visualizations

The NWB team currently maintains these visualizations of stats of NWB data on the DANDI Archive:
https://nwb-overview.readthedocs.io/en/latest/nwb-project-analytics/docs/source/code_stat_pages/dandi_nwb_stats.html

It would be great if this page could include such stats.
```
- # of dandisets that include NWB data, over time
- total size of NWB data (PBs) on DANDI
- # of NWB files on DANDI
- # of NWB dandisets, broken down by species
- # of NWB dandisets, broken down by modality
- # of NWB dandisets, broken down by dandiset size
- Also would be nice to include an interactive map of NWB data downloaded from DANDI
```

Start with the linked NWB project analytics page and inspect this repository’s existing visualization entry points; the issue names no files or tests. Done means adding the requested DANDI/NWB counts and breakdowns, plus an interactive map of downloaded NWB data, with the final scope confirmed by the project.

Written by the indexing model from the issue text.