git: ignore temporarily created build assets (#220)

Author: khanhtc1202

.gitignore

@@ -40,6 +40,11 @@ website/src/modern-cpp/zh-cn/*
 website/src/modern-cpp/en-us/*
 website/src/modern-cpp/exercises
 website/src/modern-cpp/code
+website/src/modern-cpp/about/donate.md
+website/src/modern-cpp/assets/alipay.jpg
 website/src/modern-cpp/assets/cover-2nd-en.png
 website/src/modern-cpp/assets/cover-2nd.png
-website/src/modern-cpp/assets/figures/*
+website/src/modern-cpp/assets/cover-2nd-en-logo.png
+website/src/modern-cpp/assets/cover-2nd-logo.png
+website/src/modern-cpp/assets/figures/*
+website/src/modern-cpp/assets/wechat.jpg