ci: add venv caching to speed up doc deployments

.forgejo/workflows/deploy-docs.yml

@@ -17,11 +17,25 @@ jobs:
           apt-get update
           apt-get install -y python3 python3-pip python3-venv graphviz
 
-      - name: Install Python Requirements & Compile Docs
+      - name: Cache Python Virtual Environment
+        id: cache-venv
+        uses: actions/cache@v4
+        with:
+          path: venv
+          key: ${{ runner.os }}-venv-${{ hashFiles('requirements.txt') }}
+          restore-keys: |
+            ${{ runner.os }}-venv-
+
+      - name: Install Python Requirements
+        if: steps.cache-venv.outputs.cache-hit != 'true'
         run: |
           python3 -m venv venv
           . venv/bin/activate
           pip install -r requirements.txt
+
+      - name: Compile Docs
+        run: |
+          . venv/bin/activate
           python3 build_docs.py
 
       - name: gh-pages 브랜치로 site/ 배포