Use fetch instead of pull to avoid needing to rebase gh-pages branch (

#508)

* Use `git fetch` instead of pull to avoid needing to rebase gh-pages branch

* Also add workflow-dispatch trigger for HTML build

* Add the new file path

.github/workflows/pages.yml

@@ -4,6 +4,7 @@ on:
   push:
     branches:
       - develop
+  workflow_dispatch:
 
 jobs:
 
@@ -35,11 +36,11 @@ jobs:
 
     - name: Commit to gh-pages
       run: |
-        git pull origin gh-pages
+        git fetch origin gh-pages
         git checkout gh-pages
-        git add optimade.html
         mkdir -p specification/develop
         mv optimade.html specification/develop/index.html
+        git add specification/develop/index.html
         git commit -m "Deploy develop specification to GitHub Pages: ${SHA}"
         if git diff --cached --quiet; then
             exit 0