Add 404 page and set landing-page layout (#693)

Co-authored-by: Brandon Morelli <[email protected]>

Author: reakaleek

Diff for: 404.md

@@ -0,0 +1,5 @@
+---
+layout: not-found
+---
+
+# Page not found

Diff for: docset.yml

@@ -2,7 +2,6 @@ project: 'Elastic documentation'
 
 features:
   primary-nav: true
-  landing-page: true
 
 exclude:
   - 'README.md'
@@ -61,6 +60,7 @@ toc:
   - toc: reference
   - toc: extend
   - toc: raw-migrated-files
+  - hidden: 404.md
 
 subs:
   ref:   "https://www.elastic.co/guide/en/elasticsearch/reference/current"

Diff for: index.md

@@ -1 +1,6 @@
-# Elastic documentation!!!!
+---
+navigation_title: Elastic documentation
+layout: landing-page
+---
+
+# Elastic documentation