[CI] Add check that fails if PR branch is named main

[chalin]

Our contribution guidelines ask that PRs be submitted with a branch name other than main. I'd like to suggest that we add a simple GH PR check to report this when the PR is first submitted. Otherwise, we maintainers often only find out when it's "too late", i.e., we've gone through rounds of reviews, and want to submit a fix (either via our /fix commands or manually), but can't. WDYT @open-telemetry/docs-approvers?