doc: fix scrolling glitch causing search bar to be partially hidden

With the recent update to Sphinx RTD theme 3.0.0, a 19px-high element
that used to display the version number is not visible anymore, so the
JS code that hides the upper-left logo when scrolling down needs to be
adjusted to account for this change.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>
diff --git a/doc/_static/js/custom.js b/doc/_static/js/custom.js
index 52f254b..049d327 100644
--- a/doc/_static/js/custom.js
+++ b/doc/_static/js/custom.js
@@ -16,7 +16,7 @@
         // Configuration.
 
         // The number of pixels the user must scroll by before the logo is completely hidden.
-        const scrollTopPixels = 156;
+        const scrollTopPixels = 137;
         // The target margin to be applied to the navigation bar when the logo is hidden.
         const menuTopMargin = 54;
         // The max-height offset when the logo is completely visible.