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/docs/reference/glossary/affinity.md

@@ -0,0 +1,50 @@
+---
+title: 亲和性（Affinity）
+id: affinity
+date: 2019-01-11
+full_link: zh/docs/concepts/scheduling-eviction/assign-pod-node/#affinity-and-anti-affinity
+short_description: >
+     调度程序用于确定在何处放置 Pods（亲和性）的规则
+aka:
+tags:
+- fundamental
+---
+<!--
+---
+title: Affinity
+id: affinity
+date: 2019-01-11
+full_link: /docs/concepts/scheduling-eviction/assign-pod-node/#affinity-and-anti-affinity
+short_description: >
+     Rules used by the scheduler to determine where to place pods
+aka:
+tags:
+- fundamental
+---
+-->
+
+<!--
+In Kubernetes, _affinity_ is a set of rules that give hints to the scheduler about where to place pods.
+-->
+在 Kubernetes 中，_亲和性（affinity）_是一组规则，它们为调度程序提供在何处放置 Pods 提示信息。
+<!--more-->
+<!--
+There are two kinds of affinity:
+-->
+亲和性有两种：
+<!--
+* [node affinity](/docs/concepts/scheduling-eviction/assign-pod-node/#node-affinity)
+* [pod-to-pod affinity](/docs/concepts/scheduling-eviction/assign-pod-node/#inter-pod-affinity-and-anti-affinity)
+-->
+* [节点亲和性](/zh/docs/concepts/scheduling-eviction/assign-pod-node/#node-affinity)
+* [Pod 间亲和性](/zh/docs/concepts/scheduling-eviction/assign-pod-node/#inter-pod-affinity-and-anti-affinity)
+
+<!--
+The rules are defined using the Kubernetes {{< glossary_tooltip term_id="label" text="labels">}},
+and {{< glossary_tooltip term_id="selector" text="selectors">}} specified in {{< glossary_tooltip term_id="pod" text="pods" >}}, 
+and they can be either required or preferred, depending on how strictly you want the scheduler to enforce them.
+-->
+这些规则是使用 Kubernetes {{< glossary_tooltip term_id="label" text="标签">}}（label）
+和 {{< glossary_tooltip term_id="pod" text="pods" >}} 中指定的
+{{< glossary_tooltip term_id="selector" text="选择算符">}}定义的，
+这些规则可以是必需的或首选的，这取决于你希望调度程序执行它们的严格程度。