[REF] doc: update theme & website tutorial + doc

This fixes some details and reorganize the order of the tutorial. It
also introduces a more detailed section for the SCSS customization. A
very detailed and updated version of this tutorial is being made for the
14.0 version and later.

task-2440442

closes odoo/odoo#64593

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>