CC documentation update

[ClePol]

This pull request makes several documentation updates to improve clarity, accuracy, and consistency regarding the FastSurfer-CC corpus callosum module. The main changes include updating references to published articles, standardizing file naming conventions, expanding the module description, and improving documentation formatting.

Added FastSurfer-CC citation

• Updated the FastSurfer-CC publication reference to reflect its publication in Imaging Neuroscience 2026, replacing the previous arXiv preprint citation in both the main README.md and the module documentation.
• Added a dedicated "References" section to the corpus callosum module documentation

Other doc improvements

• Standardized the file name for the 3D thickness visualization output from callosum_thickness.png to callosum.thickness.png in both the output files overview and example usage, ensuring consistency across documentation.
• Improved phrasing and formatting in the expert documentation for FastSurfer-CC, including clearer recommendations for running the tool and better code formatting for interface references.
• Expanded the module overview to provide a clearer, more comprehensive summary of the FastSurfer-CC pipeline’s steps and outputs.