Search correct language for Docusaurus docsearch (#744)

* Remove duplicate search results

Only search the current version of the docs. Right now there are duplicates because `next` is also searched.

https://github.com/facebook/Docusaurus/blob/d28b864a59fabeea45add8c090a13de7d0530de5/lib/core/Site.js#L140 does the replacement

* Add language facet

* fix missing algoliaOptions

* use only language facetfilter

website/siteConfig.js

@@ -36,6 +36,9 @@ const siteConfig = {
   algolia: {
     apiKey: '3eb9507824b8be89e7a199ecaa1a9d2c',
     indexName: 'docusaurus',
+    algoliaOptions: {
+      facetFilters: [ "lang:LANGUAGE" ] 
+    }
   },
   colors: {
     primaryColor: '#2E8555',