Rename default branch to main

[namurphy]

There has been a push in the tech industry to remove language related to slavery (specifically the use of "master" and "slave" as descriptors). As part of that push, an increasing number of projects have renamed their default branch from master to main. The GitHub page on renaming the default branch has more information on how to do this. They've made it easier to rename the default branch, since doing so will re-target open pull requests, and web requests for the old branch name will be redirected to the new branch name. There will be a notice on the webpage that provides the git commands needed to update clones of the repository on our local machines.

The steps we need to take for this include (but might not be limited to):

• Rename the default branch to main
• Change master to main in documentation guide
• Change master to main in GitHub Actions files
• Update the branch name on Read the Docs

I might be missing a few steps. We might also need to update tests using astropy-dev and numpy-dev to use main instead of master, since they might be changing default branch names too. (I know at least that Astropy is planning to.) I volunteered at the community meeting yesterday for this, so I'll assign myself to it.