Update GitHub mirror metadata

[effigies]

Is your feature request related to a problem? Please describe. To improve the GitHub crawling experience, we could export more of our local metadata to GitHub.

Describe the solution you'd like GitHub has "topics" that seem to be arbitrary tags. We can update the topics with the modalities detected in the latest snapshot.

Topics must be fully replaced and lowercase: https://docs.github.com/en/rest/repos/repos#replace-all-repository-topics

While we're at it, it would be good to update the description and homepage (https://docs.github.com/en/rest/repos/repos#update-a-repository). I would suggest that we use the dataset name for the description (falling back to "OpenNeuro dataset {accession}") and using either the OpenNeuro URL or the DOI for the homepage.

Describe alternatives you've considered We could do something like modality:MRI and leave the possibility open to do something like task:<TaskName> and other extracted features. My guess is this is over-thinking it.

Additional context