ci: update Jenkinsfile for CI/CD pipeline

Jenkinsfile

@@ -0,0 +1,12 @@
+pipeline {
+    agent any
+    environment { DOCS = '/var/www/docs' }
+    options { buildDiscarder(logRotator(numToKeepStr: '3')) }
+    stages {
+        stage('Checkout') { steps { checkout scm } }
+        stage('Deploy') {
+            when { branch 'main' }
+            steps { sh 'mkdir -p ${DOCS} && cp -r . ${DOCS}/' }
+        }
+    }
+}