Switch language name 'zh' to 'zh-cn'

This is the first step to rename 'zh' to 'zh-cn'. There are several reasons why we rename the language name.

- The upstream docsy theme changed the language name, leading to many warnings during site build;
  The side-effect is that the i18n strings are no longer working.
- We believe renaming the language is the right thing to do, because this move can make room for other variants of Chinese language, such as 'zh-tw', 'zh-sg' etc.

There would be several follow-ups to this PR, such as fixing the intra-site links, adding redirects etc.

We will lock up changes to zh/zh-cn pages for the moment, until this one gets in.

This PR is based on commit cdad0a7342.

content/zh-cn/examples/application/job/rabbitmq/Dockerfile

@@ -0,0 +1,10 @@
+# Specify BROKER_URL and QUEUE when running
+FROM ubuntu:18.04
+
+RUN apt-get update && \
+    apt-get install -y curl ca-certificates amqp-tools python \
+       --no-install-recommends \
+    && rm -rf /var/lib/apt/lists/*
+COPY ./worker.py /worker.py
+
+CMD  /usr/bin/amqp-consume --url=$BROKER_URL -q $QUEUE -c 1 /worker.py