doc: develop: Add XDG_CONFIG_HOME information for zephyrrc

The `zephyr-env.sh` script was adjusted to look for the zephyrrc file in
three locations following the [XDG Base Directory Specification]. The
documentation of the environment variables is now adjusted to reflect that
change.

[XDG Base Directory Specification]: (https://specifications.freedesktop.org/basedir-spec/latest/

)

Signed-off-by: Andreas Schuster <andreas.schuster@schuam.de>