Add archive page (#996)

Depends on elastic/docs-builder#892

See:
-
https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/996/archive
-
https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/996/#product-resources
(most right box)

Author: reakaleek

archive.md

@@ -0,0 +1,6 @@
+---
+layout: archive
+description: Browse the archived documentation for older Elastic product releases. Archived documentation is unmaintained and may be out-of-date.
+---
+
+# Documentation archive

docset.yml

@@ -68,6 +68,7 @@ toc:
   - toc: release-notes
   - toc: reference
   - toc: extend
+  - file: archive.md
   - hidden: 404.md
 
 subs: