feat(i18n): Add Simplified Chinese translation (#4379)

## Changes:

- Added `zh_CN.json` for Simplified Chinese translation.
- Modified some minor issues in README.md:
- Fixed the issue that the path docs/check-message-template.md was not
found.
- Updated the supported languages (added Greek, Italian, Simplified
Chinese and Traditional Chinese).

---------

Co-authored-by: dequejosie <83524297+dequejosie@users.noreply.github.com>

README.md

@@ -103,7 +103,7 @@ or equivalently:
 
 This will create a new build for axe, called `axe.<lang>.js` and `axe.<lang>.min.js`. If you want to build all localized versions, simply pass in `--all-lang` instead. If you want to build multiple localized versions (but not all of them), you can pass in a comma-separated list of languages to the `--lang` flag, like `--lang=nl,ja`.
 
-To create a new translation for axe, start by running `grunt translate --lang=<langcode>`. This will create a json file fin the `./locales` directory, with the default English text in it for you to translate. Alternatively, you could copy `./locales/_template.json`. We welcome any localization for axe-core. For details on how to contribute, see the Contributing section below. For details on the message syntax, see [Check Message Template](/docs/check-message-template.md).
+To create a new translation for axe, start by running `grunt translate --lang=<langcode>`. This will create a json file fin the `./locales` directory, with the default English text in it for you to translate. Alternatively, you could copy `./locales/_template.json`. We welcome any localization for axe-core. For details on how to contribute, see the Contributing section below. For details on the message syntax, see [Check Message Template](/doc/check-message-template.md).
 
 To update existing translation file, re-run `grunt translate --lang=<langcode>`. This will add new messages used in English and remove messages which were not used in English.
 
@@ -149,6 +149,10 @@ Axe-core supports the following locales. Do note that since locales are contribu
 - Polish
 - Portuguese (Brazilian)
 - Spanish
+- Greek
+- Italian
+- Simplified Chinese
+- Traditional Chinese
 
 ## Updates & Security