website: Make most of /docs/plugins into ghost pages on Extend

These pages are thoroughly obsolete. Later, we'll delete and redirect them; for
now, we'll make sure the relevant pages are front-and-center in the sidebar if
someone somehow ends up on here.

website/docs/plugins/basics.html.md

@@ -1,5 +1,5 @@
 ---
-layout: "docs"
+layout: "extend"
 page_title: "Plugin Basics"
 sidebar_current: "docs-plugins-basics"
 description: |-

website/docs/plugins/index.html.md

@@ -1,5 +1,5 @@
 ---
-layout: "docs"
+layout: "extend"
 page_title: "Plugins"
 sidebar_current: "docs-plugins"
 description: |-

website/docs/plugins/provider.html.md

@@ -1,5 +1,5 @@
 ---
-layout: "docs"
+layout: "extend"
 page_title: "Provider Plugins"
 sidebar_current: "docs-plugins-provider"
 description: |-