Add generation meta tag

Closes gh-276

Author: rwinch

src/partials/head-meta.hbs

@@ -1,5 +1,6 @@
     {{!-- Add additional meta tags here --}}
     <meta name="antora-ui-version" content="@@antora-ui-version"> {{!-- replaced by the gulp build --}}
     <meta name="version" content="{{{page.version}}}">
+    <meta name="generation" content="{{{page.componentVersion.versionSegment}}}">
     <meta name="component" content="{{{page.component.name}}}">
     <meta name="latest-version" content="{{~#if (and page.latest (eq page.url page.latest.url))}}true{{~else}}false{{~/if}}">