doc: scripts: Retrieve west path at runtime

Since west is no longer part of the Zephyr tree, we can no longer
hardcode its path anymore. Instead, use west itself to retrieve its
path, in order to point the documentation build to the correct
active west installation.
This is optional, and the documentation build will still work if west is
not installed on the system.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>