doc: ext/gh_utils: pass MAINTAINERS.yml to Maintainers

This changes to pass full path of MAINTAINERS.yml to
get_maintainer.Maintainers(). Without this, Maintainers would
use git to find the top level of Zephyr tree. This restricts
building of doc only when the build directory is under Zephyr
root. Since we have ZEPHYR_BASE in gh_utils, we can pass full
path of MAINTAINERS.yml to Maintainers() so that doc build
directory no longer has to be under Zephyr root.

Fixes #65037

Signed-off-by: Daniel Leung <daniel.leung@intel.com>