Add HF Speech Bench to Librispeech Dataset Card
_The documentation is not available anymore as the PR was closed or merged._
Adds the HF Speech Bench to Librispeech Dataset Card in place of the Papers With Code Leaderboard. Should improve usage and visibility of this leaderboard! Wondering whether this can also be done for [Common Voice 7](https://huggingface.co/datasets/mozilla-foundation/common_voice_7_0) and [8](https://huggingface.co/datasets/mozilla-foundation/common_voice_8_0) through someone with permissions? 

cc @patrickvonplaten: more leaderboard promotion!