docs: add multiversioned documentation (#344)

Documentation is now published on the `gh-pages` branch.

The branch will have a folder `docs` containing the following structure:
- `./docs` the copy from the `main` docs, but only used for the `index.html` page to do the multi-version routing.
- `./docs/main` the copy from the `main` branch docs folder
- `./docs/beta` the copy from the `beta` branch docs folder

Closes #332

.github/workflows/pages.yml

@@ -0,0 +1,44 @@
+name: Github Pages
+on:
+  push:
+    branches:
+      - main
+      - beta
+    paths:
+      - '.github/workflows/pages.yml'
+      - 'docs/**'
+
+env:
+  # Publish the main documentation in 'docs/main' folder, and the beta in 'docs/beta'
+  # Routing and version selection is handled in the main documentation.
+  DESTINATION_DIR: ${{ github.ref == 'refs/heads/main' && 'docs/main' || 'docs/beta' }}
+
+jobs:
+  release:
+    name: Release
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+          persist-credentials: false
+
+      - name: Deploy index.html
+        uses: peaceiris/actions-gh-pages@v4
+        if: github.ref == 'refs/heads/main'
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          publish_dir: ./docs
+          destination_dir: docs
+          # Subfolders docs/main and docs/beta will contain the actual versioned documentation.
+          # This is just to have the rooting properly done at the root.
+          keep_files: true
+
+      - name: Deploy versioned documentation
+        uses: peaceiris/actions-gh-pages@v4
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          publish_dir: ./docs
+          destination_dir: ${{ env.DESTINATION_DIR }}
+          keep_files: false