Feature request: benchmark description

## Request

Add a new optional property `description` to each benchmark. When present, the description is displayed below the benchmark name or below the benchmark graph.

## Rationale

If we look for example at https://zio.github.io/zio-kafka/dev/bench/ we see vague benchmark names that are derived from code names. By themselves they make sense, but _every time_ we need to look up what the benchmark actually does.

_Without the description, the benchmark result cannot be interpreted and is therefore worthless._