docs(plugin-custom-variables): add docs

[zhangzhonghe]

Related PR:
nocobase/nocobase#6916
https://github.com/nocobase/pro-plugins/pull/356

[Copilot]

Pull Request Overview

This PR adds a comprehensive usage manual for the custom variables plugin in multiple languages (zh-CN, ja-JP, fr-FR, en-US) and updates the handbook configuration accordingly.

• New documentation files added in Chinese, Japanese, French, and English covering installation, usage, and best practices.
• The handbook configuration file was updated to include a new entry for custom variables with proper titles and links.

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
docs/zh-CN/handbook/custom-varaibles/index.md	Added Chinese documentation for the custom variables plugin.
docs/ja-JP/handbook/custom-varaibles/index.md	Added Japanese documentation for the custom variables plugin.
docs/fr-FR/handbook/custom-varaibles/index.md	Added French documentation for the custom variables plugin.
docs/en-US/handbook/custom-varaibles/index.md	Added English documentation for the custom variables plugin.
docs/config/handbook.ts	Updated config to include a new handbook entry for custom variables.

Comments suppressed due to low confidence (5)

docs/zh-CN/handbook/custom-varaibles/index.md:1

• The directory name 'custom-varaibles' appears misspelled. Consider renaming it to 'custom-variables' to maintain consistency with the handbook config and other references.

# NocoBase 自定义变量插件使用手册<Badge>v2.0.0+</Badge>

docs/ja-JP/handbook/custom-varaibles/index.md:1

• The directory name 'custom-varaibles' appears misspelled. It is recommended to rename it to 'custom-variables' for consistency across the documentation.

# NocoBase 自定义变量插件使用手册<Badge>v2.0.0+</Badge>

docs/fr-FR/handbook/custom-varaibles/index.md:1

• The folder name 'custom-varaibles' seems to have a spelling error. Renaming it to 'custom-variables' would align it with the naming used elsewhere.

# NocoBase 自定义变量插件使用手册<Badge>v2.0.0+</Badge>

docs/en-US/handbook/custom-varaibles/index.md:1

• The directory name 'custom-varaibles' appears to be misspelled. It is advised to change it to 'custom-variables' to ensure consistency across the project.

# NocoBase 自定义变量插件使用手册<Badge>v2.0.0+</Badge>

docs/config/handbook.ts:1597

• The handbook config uses the link '/handbook/custom-variables', which does not match the folder name 'custom-varaibles'. Please update either the config or the folder name to ensure consistency.

link: '/handbook/custom-variables',