Add Docsearch facet attribute for search feature (#1272)

We're exploring the possibility of adding a global search feature
(including docs, blog, etc.) on the main min.io website. As part of
this, we are adding a higher-level facet to filter results based on the
given site. This is for experimental purposes. If it doesn't work as
expected, we will send a PR in the future to remove this meta attribute.

Part of miniohq/min.io#2136

Author: rushenn

source/_templates/head.html

@@ -1,4 +1,5 @@
 <!-- Algolia meta attrs -->
+<meta name="docsearch:site" content="docs"/>
 <meta name="docsearch:platform" content="{{ doc_platform }}"/>
 <meta name="docsearch:version" content="{{ doc_platform }}"/>
 <meta name="pagename" content="{{ pagename }}"/>