Update boilerplate code to use main branch by default instead of master

[Jill Cates]

Context

Github's new default branch name is "main": https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/about-branches#about-the-default-branch

Same goes for Gitlab: https://docs.gitlab.com/ee/user/project/repository/branches/default.html

Jupyterbook is still referencing "master".

Proposal

Let's update the docs and boilerplate code to reflect Github's new default branch name.

Tasks and updates

I'd be happy to take this on, but please let me know if there's someone who'd be willing to review it.

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