Add Chinese support (#13)

# Main changes and Some notices

- Chines Docs should be put into corresponding versions' folders under `i18n/zh-cn/docusaurus-plugin-content-docs`. See [translate-markdown-files](https://docusaurus.io/docs/i18n/tutorial#translate-markdown-files) for reference. And also [srs-docs](https://github.com/ossrs/srs-docs) is a project that can be used as a reference. So changed the related paths in `loop_docs*` scripts.

- The markdown files under `docs/<version>` and `i18n/zh-cn/docusaurus-plugin-content-docs/<version>` must have the same name to be referred as translated. Files inside docs are corresponding to `current` version.
For example, `docs/administration/mail-templates.md` and `i18n/zh-cn/docusaurus-plugin-content-docs/current/administration/mail-templates.md`.  Filename of `mail-templates.md` must be the same (cannot be `mail-templates.zh-cn.md` and `mail-templates.us-en.md`)
So trim the `.en-us.md` and `.zh-cn.md` to `.md` to ensure the same names inside `loop_docs*` scripts.

- Used `npx docusaurus write-translations --locale zh-cn` to do sidebar, footer, and header translations, so no need to keep `sideBarCN` anymore. [reference](https://docusaurus.io/docs/cli#docusaurus-write-translations-sitedir)

- Local Tests for `en` and `zh-cn` are separeted, run the following on local respectly to test them

    ```bash
    # test en version
    npm run start
    # test zh-cn version
    npm run start -- --locale zh-cn
    ```

- If wants to test `en` and `zh-cn` both at the same time, run build and serve

    ```
    npm run build
    npm run serve
    ```

- Added some indexed page, for example:

![Screen Shot 2023-05-26 at 13.49.34](/attachments/9eeb3d38-c309-457d-9015-053b9c5ca8d6)

![Screen Shot 2023-05-26 at 13.49.49](/attachments/fc348d17-ce4c-4c4d-9240-b69add4ef06e)

Reviewed-on: https://gitea.com/gitea/gitea-docusaurus/pulls/13
Co-authored-by: HesterG <hestergong@gmail.com>
Co-committed-by: HesterG <hestergong@gmail.com>

versioned_sidebars/version-1.19-sidebars.json

@@ -1,13 +1,16 @@
 {
-  "defaultSidebar": [
+  "docs": [
     {
-      "type": "link",
-      "label": "What is Gitea?",
-      "href": "/"
+      "type": "doc",
+      "id": "intro",
+      "label": "What is Gitea?"
     },
     {
       "type": "category",
       "label": "Installation",
+      "link": {
+        "type": "generated-index"
+      },
       "items": [
           {
           "type": "autogenerated",
@@ -18,6 +21,9 @@
     {
       "type": "category",
       "label": "Administration",
+      "link": {
+        "type": "generated-index"
+      },
       "items": [
           {
           "type": "autogenerated",
@@ -28,6 +34,9 @@
     {
       "type": "category",
       "label": "Usage",
+      "link": {
+        "type": "generated-index"
+      },
       "items": [
         {
           "type": "category",
@@ -48,6 +57,9 @@
     {
       "type": "category",
       "label": "Development",
+      "link": {
+        "type": "generated-index"
+      },
       "items": [
           {
           "type": "autogenerated",
@@ -58,6 +70,9 @@
     {
       "type": "category",
       "label": "Contributing",
+      "link": {
+        "type": "generated-index"
+      },
       "items": [
           {
           "type": "autogenerated",
@@ -68,6 +83,9 @@
     {
       "type": "category",
       "label": "Help",
+      "link": {
+        "type": "generated-index"
+      },
       "items": [
           {
           "type": "autogenerated",