Docs: fix position of search modal to avoid interface blocking at small breakpoints (#39739)

Co-authored-by: Julien Déramond <[email protected]>

Author: louismaximepiton

site/assets/scss/_search.scss

@@ -47,6 +47,7 @@
   --docsearch-muted-color: var(--bs-secondary-color);
   --docsearch-hit-shadow: none;
 
+  position: fixed;
   z-index: 2000; // Make sure to be over all components showcased in the documentation
   cursor: auto; // Needed because of [role="button"] in Algolia search modal. Remove once https://github.com/algolia/docsearch/issues/1370 is tackled.