docs: fix switchroot source link
docs: fix switchroot source link

This change updates the switchroot source relative url to an absolute url so
that it works from the readthedocs service. This change also removes
redundant switchroot.sh reference.

Closes: #1849
Approved by: cgwalters

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree