Rename `master` branch to `main`

The primary branch name `master` is a holdover from pulling the docs out of the crossplane/crossplane repo. [c/c #2045](https://github.com/crossplane/crossplane/issues/2045) tracks the eventual renaming for that repo.

### Impact of change
There are very few dependencies/automations on the branch naming. This would impact the Netlify deploy that points to `master`. It would also require updating the docs contrib guide.

Any inbound links to the master branch will be [auto redirected by Github.](https://github.blog/changelog/2020-07-17-links-to-deleted-branches-now-redirect-to-the-default-branch/)