Add intro sentence to describe Hyperledger Grid on landing page
<h1>Hyperledger Grid documentation</h1>
+ <p class="body">Grid is a platform for building supply chain solutions that include distributed ledger components.</p>
+
<p class="biglink"><a class="biglink" href="{{ pathto("introduction") }}">{% trans %}Introduction{% endtrans %}</a><br/>
<span class="linkdescr">{% trans %}meet Hyperledger Grid{% endtrans %}</span></p>
