website: move docs/index.html to docs/cli-index.html

Terraform is way bigger than the core CLI tools and the language
now, and the docs have grown accordingly. So we're adding a
global index page to help users get around the many sections of the
docs site, and bumping the CLI/core docs down so they're no longer at the
top of the hierarchy.

The "right" (as in, conceptually pure) way to do this would be to actually
create a new level of directory hierarchy in between. But that would be real
expensive and annoying — the amount of 301s and links to edit would be
monumental, and it wouldn't gain us much beyond a certain picture-straightening
satisfaction, so I'm resisting the temptation.

website/layouts/backend-types.erb

@@ -1,6 +1,6 @@
 <% wrap_layout :inner do %>
   <% content_for :sidebar do %>
-    <h4><a href="/docs/index.html">Terraform CLI</a></h4>
+    <h4><a href="/docs/cli-index.html">Terraform CLI</a></h4>
 
     <ul class="nav docs-sidenav">
       <li<%= sidebar_current("docs-backends") %>>

website/layouts/commands-providers.erb

@@ -1,6 +1,6 @@
 <% wrap_layout :inner do %>
     <% content_for :sidebar do %>
-      <h4><a href="/docs/index.html">Terraform CLI</a></h4>
+      <h4><a href="/docs/cli-index.html">Terraform CLI</a></h4>
 
       <ul class="nav docs-sidenav">
         <li<%= sidebar_current("docs-commands") %>>

website/layouts/commands-state.erb

@@ -1,6 +1,6 @@
 <% wrap_layout :inner do %>
   <% content_for :sidebar do %>
-    <h4><a href="/docs/index.html">Terraform CLI</a></h4>
+    <h4><a href="/docs/cli-index.html">Terraform CLI</a></h4>
 
     <ul class="nav docs-sidenav">
       <li<%= sidebar_current("docs-commands") %>>

website/layouts/commands-workspace.erb

@@ -1,6 +1,6 @@
 <% wrap_layout :inner do %>
   <% content_for :sidebar do %>
-    <h4><a href="/docs/index.html">Terraform CLI</a></h4>
+    <h4><a href="/docs/cli-index.html">Terraform CLI</a></h4>
 
     <ul class="nav docs-sidenav">
       <li<%= sidebar_current("docs-commands") %>>

website/layouts/docs.erb

@@ -1,6 +1,6 @@
 <% wrap_layout :inner do %>
   <% content_for :sidebar do %>
-    <h4><a href="/docs/index.html">Terraform CLI</a></h4>
+    <h4><a href="/docs/cli-index.html">Terraform CLI</a></h4>
 
     <ul class="nav docs-sidenav">
       <li<%= sidebar_current("docs-conf") %>>

website/layouts/functions.erb

@@ -1,6 +1,6 @@
 <% wrap_layout :inner do %>
   <% content_for :sidebar do %>
-    <h4><a href="/docs/index.html">Terraform CLI</a></h4>
+    <h4><a href="/docs/cli-index.html">Terraform CLI</a></h4>
 
     <ul class="nav docs-sidenav">
       <li<%= sidebar_current("docs-config-index") %>>