Update Project Structure diagram in the documentation

[h-mayorquin]

The current one is out of date I think:

https://neuroconv.readthedocs.io/en/main/_images/neuroconv_uml.svg

[CodyCBakerPhD]

Yes, that was made using a custom UML diagram -> image file software, I much prefer the integrated Mermaid approach we have been using in extensions as of late