Try to fix 'pages' CI job.

Use newer `pandoc` release (3.1.12.3) as system dependency to build documentation.

.gitlab-ci.yml

@@ -95,7 +95,10 @@ pages:
   image: continuumio/miniconda3
   script:
     # Install required 'libgl1' system dependency:
-    - apt-get update && apt-get install libgl1 pandoc -y
+    - apt-get update && apt-get install libgl1 wget -y
+    # Download and install pandoc3.1.12.3 (version must be at least (2.14.2) but less than (4.0.0)):
+    - wget https://github.com/jgm/pandoc/releases/download/3.1.12.3/pandoc-3.1.12.3-1-amd64.deb
+    - sudo dpkg -i pandoc-3.1.12.3-1-amd64.deb
     # Install faster solver 'conda-libmamba-solver' & set it as default solver:
     # - conda install -y -n base -c conda-forge conda-libmamba-solver
     # - conda config --set solver libmamba