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/quantity.md

@@ -0,0 +1,64 @@
+---
+title: 量纲（Quantity）
+id: quantity
+date: 2018-08-07
+full_link:
+short_description: >
+  使用全数字来表示较小数值或使用 SI 后缀表示较大数值的表示法。
+
+aka: 
+tags:
+- core-object
+---
+
+<!--
+title: Quantity
+id: quantity
+date: 2018-08-07
+full_link:
+short_description: >
+  A whole-number representation of small or large numbers using [SI](https://en.wikipedia.org/wiki/International_System_of_Units) suffixes.
+
+aka: 
+tags:
+- core-object
+-->
+
+<!--
+A whole-number representation of small or large numbers using SI suffixes.
+-->
+使用全数字来表示较小数值或使用 [SI](https://zh.wikipedia.org/wiki/International_System_of_Units) 后缀表示较大数值的表示法。
+
+<!--more-->
+
+<!--
+Quantities are representations of small or large numbers using a compact,
+whole-number notation with SI suffixes.  Fractional numbers are represented
+using milli units, while large numbers can be represented using kilo,
+mega, or giga units.
+
+For instance, the number `1.5` is represented as `1500m`, while the number `1000`
+can be represented as `1k`, and `1000000` as `1M`. You can also specify
+binary-notation suffixes; the number 2048 can be written as `2Ki`.
+
+The accepted decimal (power-of-10) units are `m` (milli), `k` (kilo,
+intentionally lowercase), `M` (mega), `G` (giga), `T` (terra), `P` (peta),
+`E` (exa).
+
+The accepted binary (power-of-2) units are `Ki` (kibi), `Mi` (mebi), `Gi` (gibi),
+`Ti` (tebi), `Pi` (pebi), `Ei` (exbi).
+-->
+量纲是使用紧凑的全数字表示法来表示小数值或带有国际计量单位制（SI）
+的大数值的表示法。
+小数用 milli 单位表示，而大数用 kilo、mega 或 giga 单位表示。
+
+例如，数字 `1.5` 表示为 `1500m`，
+而数字 `1000` 表示为 `1k`，`1000000` 表示为 `1M`。 
+你还可以指定二进制表示法后缀；数字 2048 可以写成 `2Ki`。
+
+公认的十进制（10 的幂数）单位是 `m`（milli）、`k`（kilo，有意小写）、
+`M`（mega）、`G`（giga）、`T`（terra）、`P`（peta）、`E`（exa）。
+
+公认的二进制（2 的幂数）单位是 `Ki` (kibi)、`Mi` (mebi)、`Gi` (gibi)、
+`Ti` (tebi)、 `Pi` (pebi)、 `Ei` (exbi)。
+