Add html_meta values

Author: stevepiercy

docs/index.md

@@ -1,9 +1,9 @@
 ---
 html_meta:
-  "description": ""
-  "property=og:description": ""
-  "property=og:title": ""
-  "keywords": ""
+  "description": "This website is the HTML documentation of Plone, an open source, enterprise-level content management system."
+  "property=og:description": "This website is the HTML documentation of Plone, an open source, enterprise-level content management system."
+  "property=og:title": "Plone 6 Documentation"
+  "keywords": "Plone 6, content management system, CMS, open source, Documentation, Volto, Classic UI, frontend, backend, plone.restapi, plone.api"
 ---
 
 (index-label)=