[FIX] website: allow to remove jumbotron bg color

In the 'banner' snippet, there is a column using a white jumbotron
component. Those comes with a default background color so when using the
website builder to remove the white background, it did not go away;
it was replaced with the default background color (some gray).

This commit sets the default background color of jumbotron components to
'transparent'.

task-1889341

closes odoo/odoo#30099

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

@@ -29,3 +29,7 @@ $font-family-sans-serif: nth($o-theme-fonts, $o-theme-font-number);
 
 $headings-font-family: nth($o-theme-fonts, $o-theme-headings-font-number);
 $headings-color: color('h1');
+
+// Jumbotron
+
+$jumbotron-bg: transparent !default;