Set up mdbook in github pages (#26367)

.github/workflows/docs.yml

@@ -0,0 +1,52 @@
+name: Deploy documentation
+
+on:
+    push:
+        branches: [develop]
+    workflow_dispatch: {}
+
+jobs:
+    docs:
+        name: GitHub Pages
+        runs-on: ubuntu-latest
+        steps:
+            - name: Fetch element-desktop
+              uses: actions/checkout@v4
+              with:
+                  repository: vector-im/element-desktop
+                  path: element-desktop
+
+            - name: Fetch element-web
+              uses: actions/checkout@v4
+              with:
+                  path: element-web
+
+            - name: Fetch matrix-react-sdk
+              uses: actions/checkout@v4
+              with:
+                  repository: matrix-org/matrix-react-sdk
+                  path: matrix-react-sdk
+
+            - name: Setup mdBook
+              uses: peaceiris/actions-mdbook@v1
+              with:
+                  mdbook-version: "0.4.10"
+
+            - name: Install mdbook-combiner
+              run: cargo install mdbook-combiner
+
+            - name: Build docs
+              run: |
+                  mkdir docs
+                  mv element-desktop/docs docs/element-desktop
+                  mv element-web/docs docs/element-web
+                  mv matrix-react-sdk/docs docs/matrix-react-sdk
+                  mdbook-combiner -m docs
+                  cp element-web/book.toml .
+                  mdbook build
+
+            - name: Deploy to gh pages
+              uses: peaceiris/actions-gh-pages@373f7f263a76c20808c831209c920827a82a2847 # v3.9.3
+              with:
+                  github_token: ${{ secrets.GITHUB_TOKEN }}
+                  publish_dir: ./book