Fix docs build: disable GUI to avoid OpenGL dependency

Author: JDRomano2

.github/workflows/build-docs.yml

@@ -52,7 +52,7 @@ jobs:
         run: |
           python -m pip install --upgrade pip
           pip install sphinx furo numpydoc breathe myst-parser
-          pip install -e .
+          CMAKE_ARGS="-DBUILD_GUI=OFF" pip install -e .
 
       - name: Build documentation (Doxygen + Sphinx)
         working-directory: docs