Comment out julia-actions/cache in documentation CI job

Seems to be causing an error with Python in plots_post_processing at the
moment. Don't know why. Maybe after the cache empties/resets somehow we
can add the caching back again?

.github/workflows/documentation.yml

@@ -15,7 +15,7 @@ jobs:
       - uses: julia-actions/setup-julia@latest
         with:
           version: '1.10'
-      - uses: julia-actions/cache@v2
+      #- uses: julia-actions/cache@v2
       - name: Install dependencies
         run: |
           # Version 3.9.0 of matplotlib causes an error with PyPlot.jl, so pin