Add forum to site header
<a class="header__logo white" href="{{ config.base_url }}">Zola</a>
<a class="white" href="{{ get_url(path="./documentation/_index.md") }}" class="nav-link">Docs</a>
<a class="white" href="{{ get_url(path="./themes/_index.md") }}" class="nav-link">Themes</a>
+ <a class="white" href="https://zola.discourse.group/" class="nav-link">Forum</a>
<a class="white" href="https://github.com/getzola/zola" class="nav-link">GitHub</a>
<div class="search-container">
