Update nav-breadcrumbs to use dedicated example project

Switch from quarto.org URL to local breadcrumbs example with matching
sidebar structure (Tutorials > Tutorial Landing). Uses darkly + custom
SCSS to make breadcrumb background transparent in dark mode.

# Conflicts:
#	docs/websites/images/nav-breadcrumbs-dark.png
#	docs/websites/images/nav-breadcrumbs.png

Author: cderv