Docs: make sidebar section titles clickable

Override the TeXt theme's _includes/sidebar/toc.html, which rendered each
top-level nav item as plain text. Turn the title into a link when the nav
item has a url, with active-state handling.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: hsyl20

_includes/sidebar/toc.html

@@ -0,0 +1,46 @@
+{%- comment -%}
+  Local override of the TeXt theme's sidebar TOC.
+
+  Upstream renders each top-level item as plain, unclickable text:
+    <li class="toc-h1">{{ _item.title }}</li>
+  Our navigation gives every section (Home, Explanation, ...) a `url`, so we
+  turn the title into a link when a `url` is present, and mark it active when
+  it is the current page. Items without a `url` keep the plain header style.
+{%- endcomment -%}
+{%- if page.sidebar.nav -%}
+  <div class="sidebar-toc">
+    {%- assign _sidebar_nav = site.data.navigation[page.sidebar.nav] -%}
+    {%- if _sidebar_nav -%}
+    <ul class="toc toc--navigator">
+      {%- for _item in _sidebar_nav -%}
+        {%- if _item.url -%}
+          {%- include snippets/get-nav-url.html path=_item.url -%}
+          {%- assign _nav_url = __return -%}
+          {%- include snippets/get-nav-url.html path=page.url -%}
+          {%- assign _page_url = __return -%}
+          {%- if _nav_url == _page_url -%}
+            <li class="toc-h1 active"><a href="{{ _nav_url }}">{{ _item.title }}</a></li>
+          {%- else -%}
+            <li class="toc-h1"><a href="{{ _nav_url }}">{{ _item.title }}</a></li>
+          {%- endif -%}
+        {%- else -%}
+          <li class="toc-h1">{{ _item.title }}</li>
+        {%- endif -%}
+        {%- if _item.children -%}
+          {%- for _child in _item.children -%}
+            {%- include snippets/get-nav-url.html path=_child.url -%}
+            {%- assign _nav_url = __return -%}
+            {%- include snippets/get-nav-url.html path=page.url -%}
+            {%- assign _page_url = __return -%}
+            {%- if _nav_url == _page_url -%}
+              <li class="toc-h2 active"><a href="{{ _nav_url }}">{{ _child.title }}</a></li>
+            {%- else -%}
+              <li class="toc-h2"><a href="{{ _nav_url }}">{{ _child.title }}</a></li>
+            {%- endif -%}
+          {%- endfor -%}
+        {%- endif -%}
+      {%- endfor -%}
+      </ul>
+    {%- endif -%}
+  </div>
+{%- endif -%}