Add module docstring for experiments module

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Co-Authored-By: Claude <[email protected]>

Author: louismagowan

causalpy/experiments/__init__.py

@@ -11,3 +11,4 @@
 #   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 #   See the License for the specific language governing permissions and
 #   limitations under the License.
+"""Quasi-experimental designs for causal inference."""