EthicalAds: update CSS selector for Material for MkDocs (#579)

The ad was not being displayed correctly because we the CSS was
incorrect on pages that are post (Material for MkDocs has a blog plugin
now)

## Regular page (same as before)


![Screenshot_2025-05-14_13-05-37](https://github.com/user-attachments/assets/7dd6765f-aadb-4921-9c99-aff2865af386)

## Blog post


![Screenshot_2025-05-14_13-05-48](https://github.com/user-attachments/assets/0c21d9cb-494e-45c3-9415-48f921c087ac)


Closes #578

Author: humitos

src/ethicalads.js

@@ -119,7 +119,9 @@ export class EthicalAdsAddon extends AddonBase {
           knownPlacementFound = true;
         }
       } else if (docTool.isMaterialMkDocsTheme()) {
-        selector = ".md-sidebar__scrollwrap";
+        // Detect the left navbar if it's not hidden or grab the navbar from a post page
+        selector =
+          ".md-sidebar--primary:not([hidden]) > .md-sidebar__scrollwrap, .md-sidebar--post > .md-sidebar__scrollwrap";
         element = document.querySelector(selector);
 
         if (this.elementAboveTheFold(element)) {