Consider changing the name of our primary branch from `master` to `main`

[ericcarraway]

Many communities, both on GitHub and in the wider Git community, are considering renaming the default branch name of their repository from master.

• On GitHub, main has been the default branch name for new repositories since October 2020.
• GitLab has a release planned for this month which will do the same.
• Most repositories in the W3 organization use main

850 W3C Repositories on 2021-04-20

Name	Count
main	271
master	266
gh-pages	191
other	4
archived status	118

[adrianholovaty]

Thanks for making this suggestion. I've just made the change. Apparently it temporarily broke our GitHub Pages installation, but I believe that's being worked out as caches get cleared.