PreviewDocs

How to test:

export BUILDKITE_COMMIT="c8b601a225"
export BUILDKITE_BUILD_CREATOR="zzak"
export BUILDKITE_REPO="https://github.com/rails/rails.git"
export BUILDKITE_BUILD_NUMBER="60"
export BUILDKITE_BUILD_URL="https://buildkite.com/rails/docs-preview/builds/60"
export BUILDKITE_BRANCH="preview_docs"
export BUILDKITE_MESSAGE="commit message"
export BUILDKITE_PULL_REQUEST="42"
bundle exec rake preview_docs
open preview/index.html