feat(nav-v2): add Extension points and Account isolated sections; remove back arrow

Adds two isolated nav sections (not shown in top bar):
- Extension points (extend + kibana/logstash/beats/elasticsearch/integrations)
- Account and preferences (cloud-account)

Removes the back arrow from isolated sections — the UX for returning
from an island is still under design review.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: theletterf

config/navigation-v2.yml

@@ -2861,3 +2861,21 @@ nav:
     url: /reference/
     children:
     - toc: docs-content://reference
+
+  - section: Extension points
+    url: /extend/
+    isolated: true
+    children:
+    - toc: docs-content://extend
+      children:
+      - toc: kibana://extend
+      - toc: logstash://extend
+      - toc: beats://extend
+      - toc: elasticsearch://extend
+      - toc: integrations://extend
+
+  - section: Account and preferences
+    url: /cloud-account/
+    isolated: true
+    children:
+    - toc: docs-content://cloud-account

src/Elastic.Documentation.Site/Navigation/_TocTree.cshtml

@@ -71,17 +71,6 @@
 	<div class="pages-nav-menu min-h-0 min-w-0 flex-1 overflow-y-auto">
 	@if (Model.IsNavV2)
 	{
-		@if (Model.IsIsolatedSection)
-		{
-			<a href="/"
-			   @Model.Htmx.GetNavHxAttributes(true, null)
-			   class="flex items-center gap-2 px-4 py-3 text-sm font-semibold text-ink-light hover:text-blue-elastic border-b border-grey-20">
-				<svg xmlns="http://www.w3.org/2000/svg" fill="none" viewBox="0 0 24 24" stroke-width="2" stroke="currentColor" class="w-4 h-4" aria-hidden="true">
-					<path stroke-linecap="round" stroke-linejoin="round" d="M10.5 19.5 3 12m0 0 7.5-7.5M3 12h18"/>
-				</svg>
-				Back to Docs
-			</a>
-		}
 		@* Root for V2-only CSS; JS still uses [data-nav-v2]. Scope overrides: .docs-sidebar-nav-v2 … *@
 		<nav class="docs-sidebar-nav-v2" data-nav-v2 @if (Model.SectionUrl is not null) { <text>data-section-url="@Model.SectionUrl"</text> }>
 			<ul class="docs-sidebar-nav-v2__tree block" id="nav-tree">