Fix GH-pages deployment (#60)

* deploy_ghpages: fix issue with sphinx version leading to crash of action

* adapted trigger for ghpages deployment

.github/workflows/deploy_ghpages.yml

@@ -1,11 +1,12 @@
 name: Docs Build & Deployment
 
 on: 
-  push:
+  pull_request:
     branches:
-    - manual_ghp_update
+    - main
     paths:
     - 'docsrc/**'
+    - '.github/workflows/deploy_ghpages.yml'
   release:
     types: [published]
 
@@ -30,12 +31,12 @@ jobs:
     - name: Install dependencies
       run: |
            python -m pip install --upgrade pip
-           python -m pip install sphinx==1.8.0
            python -m pip install sphinx_rtd_theme
            python -m pip install numpydoc
            python -m pip install sphinx-gallery
            python -m pip install sphinxcontrib-httpdomain
            python -m pip install m2r2
+           python -m pip install sphinx==1.8.4
            sudo apt install texlive-extra-utils
            sudo apt-get install texlive-latex-extra
            python -m pip install .