Fix site_url to point to GitHub Pages

Update site_url from GitHub repo URL to the correct GitHub Pages URL
at osipi.github.io/TF2.2_OSIPI-ASL-toolbox/

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: ltorres6

mkdocs.yml

@@ -1,6 +1,6 @@
 # Project information
 site_name: TF2.2_OSIPI-ASL-toolbox
-site_url: https://github.com/OSIPI/TF2.2_OSIPI-ASL-toolbox
+site_url: https://osipi.github.io/TF2.2_OSIPI-ASL-toolbox/
 site_author: OSIPI
 site_description: >-
   OSIPI aims to develop resources and best practices for the analysis of perfusion imaging data and interoperability of the analysis results.