[DOCS] Add TOC to landing page (elastic#161411)

### Summary
- Adds the TOC to the Kibana docs landing page. Removes the right sidebar from the landing page.
- Removes the "View all Elastic docs" link from the bottom of the landing page

(cherry picked from commit b7b198e)

docs/index.asciidoc

@@ -7,6 +7,8 @@ include::{docs-root}/shared/versions/stack/{source_branch}.asciidoc[]
 
 include::{docs-root}/shared/attributes.asciidoc[]
 
+include::landing-page.asciidoc[]
+
 include::user/index.asciidoc[]
 
 include::accessibility.asciidoc[]

docs/index-custom-title-page.html → docs/landing-page.asciidoc

@@ -1,3 +1,4 @@
+++++
 <style>
   * {
     box-sizing: border-box;
@@ -281,4 +282,19 @@ <h4 class="mt-3">
   </div>
 </div>
 
-<p class="my-4"><a href="https://www.elastic.co/guide/index.html">View all Elastic docs</a></p>
+<script>
+window.addEventListener("DOMContentLoaded", (event) => {
+  const left_col = document.getElementById("left_col")
+  left_col.classList.remove('col-0')
+  left_col.classList.add("col-12", "col-md-4", "col-lg-3", "h-almost-full-md", "sticky-top-md")
+  const right_col = document.getElementById("right_col")
+  right_col.classList.add('d-none')
+  const middle_col = document.getElementById("middle_col")
+  middle_col.classList.remove("col-lg-7")
+  middle_col.classList.add("col-lg-9", "col-md-8")
+  const toc = middle_col.getElementsByClassName("toc")[0]
+  toc.remove()
+  left_col.appendChild(toc);
+});
+</script>
+++++