[doc] fix readthedocs build (#46929)

The main sidebar without caching version exceeds timed out limit on
readthedocs, but the caching version makes it more complicated for local
build.

Use the cache version of the sidebar for now to unblock. I'm requesting
bigger machines from readthedocs in the mean time, to unify the build
going forward.

Test:
- CI

Signed-off-by: can <can@anyscale.com>

doc/source/_templates/main-sidebar-readthedocs.html

@@ -0,0 +1,3 @@
+<nav id="main-sidebar" class="bd-docs-nav bd-links" aria-label="{{ _('Section Navigation') }}">
+  <div class="bd-toc-item navbar-nav">{{ cached_toctree }}</div>
+</nav>

doc/source/conf.py

@@ -323,7 +323,11 @@ def render_svg_logo(path):
 }
 
 html_sidebars = {
-    "**": ["main-sidebar"],
+    "**": [
+        "main-sidebar-readthedocs"
+        if os.getenv("READTHEDOCS") == "True"
+        else "main-sidebar"
+    ],
     "ray-overview/examples": [],
 }