DISCUSS Rename master to main?
The default for new repositories is now "main", so renaming might make contributing less confusing to newcomers.

scikit-learn have made this move (see [issue](https://github.com/scikit-learn/scikit-learn/issues/17595) and [PR](https://github.com/scikit-learn/scikit-learn/pull/19237/files)),  and from https://github.com/github/renaming it doesn't look too disruptive:

> Renaming a branch will:
> 
> - Re-target any open pull requests
> - Update any draft releases based on the branch
> - Move any branch protection rules that explicitly reference the old name
> - Update the branch used to build GitHub Pages, if applicable
> - Show a notice to repository contributors, maintainers, and admins on the repository homepage with instructions to update local copies of the repository
> - Show a notice to contributors who `git push` to the old branch
> - Redirect web requests for the old branch name to the new branch name
> - Return a "Moved Permanently" response in API requests for the old branch name
> 

From the penultimate bullet point above, it seems that existing links in the docs wouldn't break even if we didn't update them, but I think the
```
  push:
    branches: [master]
```
part of the CI workflows would still need renaming.