[Docs] Update organisation .github to reflect focus on vizarr

**What needs updating?**  
Once the [biongff-viewer](https://github.com/BioNGFF/biongff-viewer) has been archived and redirected to [vizarr](https://github.com/BioNGFF/vizarr), the BioNGFF organisation needs to be updated to reflect the focus on Vizarr. 

**Where should this be updated?**  
On [.github](https://github.com/BioNGFF/.github)

