Add package documentation

[meling]

The revive linter (which runs as part of GitHub's workflow) complains because documentation is missing, resulting in red (non-passing) tests. Some of them may only need to be a single sentence.

% revive ./...
cmd/hotstuff/main.go:1:1: should have a package comment
metrics/types/event.go:1:1: should have a package comment
leaderrotation/carousel.go:1:1: should have a package comment
hotstuff.go:1:1: should have a package comment
internal/proto/hotstuffpb/convert.go:1:1: should have a package comment
synchronizer/synchronizer.go:1:1: should have a package comment
cmd/plot/main.go:1:1: should have a package comment
internal/cli/root.go:1:1: should have a package comment
twins/generator.go:1:1: should have a package comment