doc: Add guide showing auto-login on a specific TTY to the cookbook.

doc: Add guide showing auto-login on a specific TTY to the cookbook.

This is a follow-up to the discussion in <https://bugs.gnu.org/48974>.

* doc/guix-cookbook.texi (System Configuration): Add a brief guide that
explains auto login a user to one TTY.

* doc/guix.texi (System Services): Add an texinfo anchor, so that the cookbook
entry "Auto Login a User to a Specific TTY" can refer back to the precise
point that the GNU Guix Reference manual.  Also add a reference to the
cookbook that shows how to auto login a specific user to one TTY.

Signed-off-by: Leo Prikler <leo.prikler@student.tugraz.at>