doc: derive Doxygen PROJECT_NUMBER from CMakeLists.txt version

Extract the project version from the top-level CMakeLists.txt in the
Gitea Actions pages workflow and pass it to Doxygen via the
MUSRFIT_VERSION env var, so musrfit_dox.cfg's PROJECT_NUMBER no
longer needs to be kept in sync by hand.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitea/workflows/deploy-pages.yml

@@ -0,0 +1,50 @@
+name: Build and Deploy Documentation
+
+on:
+  push:
+    branches:
+      - root6
+  workflow_dispatch:
+
+jobs:
+  build-and-deploy:
+    runs-on: ubuntu-latest
+    container:
+      image: gitea.psi.ch/images/alpine-jupyterbook
+    steps:
+
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Install system dependencies
+        run: |
+          apk add doxygen graphviz
+
+      - name: Extract project version from CMakeLists.txt
+        run: |
+          VERSION=$(grep -oP 'project\(\s*musrfit\s+VERSION\s+\K[0-9]+\.[0-9]+\.[0-9]+' CMakeLists.txt)
+          echo "MUSRFIT_VERSION=$VERSION" >> $GITHUB_ENV
+
+      - name: Build musrfit-tech-docu
+        run: |
+          cd doc
+          mkdir technical
+          doxygen musrfit_dox.cfg
+          doxygen musredit_qt6_dox.cfg
+          doxygen mupp_qt6_dox.cfg
+          doxygen musrStep_qt6_dox.cfg
+          doxygen musrWiz_qt6_dox.cfg
+
+      - name: Configure Git
+        run: |
+          git config --global user.name "Gitea Actions"
+          git config --global user.email "actions@gitea.local"
+
+      - name: Push to gitea-pages branch
+        run: |
+          git checkout --orphan gitea-pages
+          git reset --hard
+          cp -r ./doc/technical/html/* .
+          git add .
+          git commit -m "Deploy site"
+          git push --verbose -f https://${{secrets.GITHUB_TOKEN}}@gitea.psi.ch/${{ github.repository }}.git gitea-pages

doc/musrfit_dox.cfg

@@ -38,7 +38,7 @@ PROJECT_NAME           = musrfit
 # could be handy for archiving the generated documentation or if some version
 # control system is used.
 
-PROJECT_NUMBER         = 1.9.10
+PROJECT_NUMBER         = $(MUSRFIT_VERSION)
 
 # Using the PROJECT_BRIEF tag one can provide an optional one line description
 # for a project that appears at the top of each page and should give viewer a