[IMP] website: allow mega menu transparency

This PR allows the user to put a transparency effect on the mega menu.
The transparency effect did not work on the mega menu because
the container that contains the mega menu did not have transparency.
Note that the mega menu templates use a default background color
since [1] to not be transparent by default because of this commit.

[1]: 967d21a

task-2623335

closes odoo/odoo#77871

Related: odoo/upgrade#2929
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>

addons/website/static/src/scss/website.scss

@@ -1193,6 +1193,7 @@ header {
     margin-top: 0;
     border-radius: 0;
     background-clip: unset; // Remove the 1px gap introduced by BS4
+    background-color: transparent; // In order to allow the mega menu to be transparent
 
     .container, .container-fluid {
         // Need to reforce those because they are removed since its a container