Improve descriptions of node types

The descriptions of each node type in [node_type_handlers.py](https://github.com/Ivareh/pg-explain-optimize-dashboard/blob/main/db-optimize-logger/app/node_type_handlers.py) are minimal; they could be more accurate and detailed.

[Pev2](https://github.com/dalibo/pev2) provides better descriptions—if we credit them, perhaps we could ask permission to use some of theirs.