ci: doc-build: Pull 'bsim' west project

This commit updates the documentation build workflow to pull in the
'bsim' west project (with its submanifest) so that the build process
does not fail due to a manifest import failure.

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>
Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>