Combine the chicken and 2 teaspoons of the salt with 2 quarts of water in a Dutch oven. Cover and bring to a boil. Reduce the heat to medium and add the pepper, onion, and celery leaves. Simmer the chicken until the meat falls away from the bone. Remove the chicken and onion from the pot, and strain out the celery leaves. Discard the onion and celery leaves, and set the chicken aside to cool. When the chicken is cool enough to handle, remove the meat from the bones and cut into 1-inch pieces. Set aside. Discard the skin and bones. Meanwhile, raise the heat to high and bring the broth to a boil. Combine the flour, baking soda, and the remaining ½ teaspoon of salt in a medium bowl. Cut in the shortening with a fork and knife or a pastry cutter until the mixture resembles coarse crumbs. Gradually stir in the milk until the dough is moist. Turn the dough out onto a floured surface and lightly knead for about 5 minutes. Roll out to ½-inch thickness and cut into 1½-inch squares. Drop the pieces of dough into the boiling broth. Reduce the heat to low, cover, and cook for 10 minutes. Stir gently. Add the chicken back to the pot, stir, and cook until the chicken is heated through, about 8 minutes. Serve warm.