[FIX] web: discard unnecessary scrollbar in kanban examples

Steps:
- Open Project
- Create New Project
- Select the blank new project and click on see examples below the stage.
- This also appears in the existing project where you want to add more stages.
- A Kanban Examples popup view will open.

Issue:
- A scrollbar appears on the bottom left due to the long name of the example.

Cause:
- Due to the overflow property, when the name is long, it will automatically
create a scrollbar, making it look weird.

Fix:
- Add overflow-x visible in notebook It will counterbalance the overflow-x: auto;
rule used in the general (aka. horizontal) use case, and force the overflowing
element to take the space it needs, adjusting the headers column's width accordingly.

closes odoo/odoo#140590

Task: 3422009
X-original-commit: 4fff4fbb308bf411603cfc239e9f651598d0de78
Signed-off-by: Pierre Paridans (app) <app@odoo.com>
Signed-off-by: Xavier Bol (xbo) <xbo@odoo.com>

addons/web/static/src/core/notebook/notebook.scss

@@ -55,6 +55,10 @@
     }
 
     &.vertical {
+        .o_notebook_headers {
+            overflow-x: visible;
+        }
+
         .nav {
             width: max-content;
             border-bottom-color: transparent; // removing the width has weird side effect due to the negative margin