docs: Add comprehensive module-level docstring to pyasl.modules

PyASL/pyasl/modules/__init__.py

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+"""
+ASL Processing Modules
+======================
+
+This package contains processing modules for Arterial Spin Labeling (ASL) MRI data.
+
+The modules are organized by their source pipeline:
+
+**ASLtbx** - SPM-based ASL processing pipeline
+    Includes coregister, create_mask, perfusion_quantify, realign, 
+    reset_orientation, and smooth modules.
+
+**DL-ASL** - Deep learning-based ASL processing
+    Provides build_mask and denoise_cbf for AI-powered processing.
+
+**MRICloud** - Cloud-based ASL quantification pipeline
+    Offers comprehensive quantification tools including calculate_CBF, calculate_M0,
+    calculate_diffmap, coreg_mpr, multidelay variants, read_mpr, rescale,
+    and t1roi_CBFaverage.
+
+**Oxford ASL** - BASIL-based ASL analysis
+    Features asl_run and asl_split_m0 for model-based analysis.
+
+**Preclinical** - Modules for preclinical ASL data
+    A complete suite for animal imaging including abs_t1fit, brain_mask,
+    cbf_relative, compute_m0, control_label_split, diff_image, loader functions
+    for Bruker and NIfTI formats, motion_check, slice_pld_adjust, and 
+    steady_state_trim.
+
+**Utilities**
+    Common functionality like save_outputs.
+
+Each module can be used independently or combined to build custom processing
+pipelines tailored to your specific ASL acquisition and analysis needs.
+
+Example Usage
+-------------
+You can import specific modules as needed:
+
+    from pyasl.modules import asltbx_coregister
+    from pyasl.modules import mricloud_calculate_CBF
+
+See Also
+--------
+pyasl.pipelines : Ready-to-use processing pipelines
+pyasl.utils : Data handling utilities
+"""