[IMP] Website Themes - Building blocks

Rework of the whole page in order to organized / categorized the content better + Addition from the PSBE Designers Wiki

[FIX] Website Themes - Building block : Fix typo

[FIX] Corrections after review

X-original-commit: 062b4c8

[FIX] Resolve conflicts

closes #12523

X-original-commit: b3e7734
Signed-off-by: Brandon Mercier (bram) <bram@odoo.com>

Author: bram-odoo