docs: update document according to the recent update

build-docs.sh

@@ -2,21 +2,28 @@
 # Description:
 #   This script is used to build the CEPCSW docs.
 
+function install-sphinx() {
+    if [ ! -d "venv" ]; then
+        python -m venv venv
+        source venv/bin/activate
+        pip install -r requirements.txt
+    else
+        source venv/bin/activate
+    fi
+}
+
 function check-sphinx() {
     which sphinx-build >& /dev/null
 }
 
 function run-build-docs() {
     pushd docs
-    if [ ! -d "venv" ]; then
-        python -m venv venv
-        source venv/bin/activate
-        pip install -r requirements.txt
-    fi
     make html
     popd
 }
 
+install-sphinx
+
 if check-sphinx; then
     run-build-docs
 else