add slack

Author: yzh119

_includes/header.html

@@ -24,6 +24,7 @@
             {%- endif -%}
           {%- endfor -%}
           <a class="page-link" href="https://docs.flashinfer.ai/">Documentation</a>
+	  <a class="page-link" href="https://join.slack.com/t/flashinfer/shared_invite/zt-2r93kj2aq-wZnC2n_Z2~mf73N5qnVGGA">Slack</a>
           <a class="page-link" href="https://arxiv.org/abs/2501.01005">Paper</a>
           <a class="page-link" href="https://github.com/flashinfer-ai/flashinfer">Github</a>
         </div>