doc: ext: link-roles: fix crash when west is available but not used
doc: ext: link-roles: fix crash when west is available but not used

It should be possible to submit quick documentation fixes without
depending on a complete west workspace. Fixes the following crash:

Exception occurred:
  File "west/src/west/util.py", line 82, in west_topdir
    raise WestNotFound('Could not find a West workspace '
west.util.WestNotFound: Could not find a West workspace in this
                        or any parent directory

Fixes ae8dd148874a ("doc: ext: link-roles: allow non-default baseurl")

Signed-off-by: Marc Herbert <77e7c7d746b52c6eb01119b65a031ae1f650246d@intel.com>

Vudentz/zephyr,galak/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr