few fixes in docs and added more detailed description for raven

Author: dkazanc

docs/source/_static/auto_images_methods/data_stripes_added_proj.png

@@ -58,6 +58,16 @@ @article{titarenko2010analytical
   year={2010},
   publisher={Elsevier}
 }
+@article{raven1998numerical,
+  title={Numerical removal of ring artifacts in microtomography},
+  author={Raven, Carsten},
+  journal={Review of scientific instruments},
+  volume={69},
+  number={8},
+  pages={2978--2980},
+  year={1998},
+  publisher={AIP Publishing}
+}
 @article{vo2015radial,
   title={Radial lens distortion correction with sub-pixel accuracy for X-ray micro-tomography},
   author={Vo, Nghia T and Atwood, Robert C and Drakopoulos, Michael},

docs/source/_static/auto_images_methods/data_stripes_added_sino.png

@@ -0,0 +1,52 @@
+.. method_raven_filter:
+
+Raven filter
+^^^^^^^^^^^^^
+
+**Description**
+
+Raven filter is a part of the module :doc:`../../api/httomolibgpu.prep.stripe` for data correction. Raven filter is based on a FFT filter applied in sinogram space to minimise stripes leading to `Ring Artifacts <https://radiopaedia.org/articles/ring-artifact-2?lang=gb>`_ in reconstruction. See more about the filter in the original publication :cite:`raven1998numerical`.
+
+**Where and how to use it:**
+
+Raven filter can be used in pre-processing, i.e., before the reconstruction to remove the stripes present in the data.
+
+**What are the adjustable parameters:**
+
+Most important parameters are :code:`uvalue` and :code:`nvalue` that define the shape of the filter, as well as, :code:`vvalue`, the number of rows to be applied to the filter.
+
+**Practical example:**
+
+In this example we demonstrate how to apply Raven filter to the normalised projection data.
+
+.. warning:: As with any pre-processing tool, one needs to be careful in applying Raven filter to data. Sub-optimal parameters can potentially lead to other artifacts introduced in the reconstruction and blurring.
+.. list-table::
+
+
+    * - .. figure:: ../../_static/auto_images_methods/data_stripes_added_sino.png
+
+           Input (sinogram view) with the stripes added
+
+      - .. figure:: ../../_static/auto_images_methods/data_stripes_added_proj.png
+
+           Input (projection view) with the stripes added
+
+    * - .. figure:: ../../_static/auto_images_methods/raven_filter_sino.png
+
+           After applying Raven filter (sinogram)
+
+      - .. figure:: ../../_static/auto_images_methods/raven_filter_proj.png
+
+           After applying Raven filter (projection)
+
+    * - .. figure:: ../../_static/auto_images_methods/raven_filter_res_sino.png
+
+           Sinogram view of absolute residual between input and output
+
+      - .. figure:: ../../_static/auto_images_methods/raven_filter_res_proj.png
+
+           Projection view of absolute residual between input and output
+
+
+
+