Add Slack community link to the docs site footer.
RELNOTES:
<li><a href="{{ site.blog_site_url }}">Blog</a></li>
<li><a href="https://github.com/bazelbuild/bazel">GitHub</a></li>
<li><a href="https://groups.google.com/forum/#!forum/bazel-discuss">Discussion group</a></li>
+ <li><a href="https://slack.bazel.build">Slack</a></li>
</ul>
</div>
</div>
